http-client-java, corner case of client without op or group (#4358)

Found when trying to write subclass as 
```ts
@client({
  name: "ContosoClient",
  service: Cadl.ContosoServer,
})
namespace Cadl.ContosoServer {
  @client({
    name: "ContosoSubClient",
    service: Cadl.ContosoServer,
  })
  namespace SubClient {
    @client({
      name: "ContosoSubSubClient",
      service: Cadl.ContosoServer,
    })
    @route("/contoso/")
    interface ServerOp {
      get(@path(#{ allowReserved: true }) group: string): OkResponse | NoContentResponse;
    }
  }
}
```

<-- this likely not the correct way to write subclient (as Client be
very different from subclient from nested namespace), but at least
emitter should not throw

After this PR, above tsp would result in `ContosoSubSubClient` client,
as this is the only one containing operation.

packages/http-client-java/emitter/src/code-model-builder.ts

@@ -199,13 +199,12 @@ export class CodeModelBuilder {
     }
 
     const service = listServices(this.program)[0];
-    const serviceNamespace = service.type;
+    if (!service) {
-    if (serviceNamespace === undefined) {
+      throw Error("TypeSpec for HTTP must define a service.");
-      throw Error("Cannot emit yaml for a namespace that doesn't exist.");
     }
-    this.serviceNamespace = serviceNamespace;
+    this.serviceNamespace = service.type;
 
-    this.namespace = getNamespaceFullName(serviceNamespace) || "Azure.Client";
+    this.namespace = getNamespaceFullName(this.serviceNamespace) || "Azure.Client";
     // java namespace
     const javaNamespace = this.getJavaNamespace(this.namespace);
 
@@ -219,9 +218,9 @@ export class CodeModelBuilder {
     };
 
     // init code model
-    const title = this.options["service-name"] ?? serviceNamespace.name;
+    const title = this.options["service-name"] ?? this.serviceNamespace.name;
 
-    const description = this.getDoc(serviceNamespace);
+    const description = this.getDoc(this.serviceNamespace);
     this.codeModel = new CodeModel(title, false, {
       info: {
         description: description,
@@ -230,7 +229,7 @@ export class CodeModelBuilder {
         default: {
           name: title,
           description: description,
-          summary: this.getSummary(serviceNamespace),
+          summary: this.getSummary(this.serviceNamespace),
           namespace: this.namespace,
         },
         java: {

packages/http-client-java/generator/http-client-generator-core/src/main/java/com/microsoft/typespec/http/client/generator/core/mapper/ClientMapper.java

@@ -192,9 +192,11 @@ public class ClientMapper implements IMapper<CodeModel, Client> {
             } else {
                 // service client
                 ServiceClient serviceClient = Mappers.getServiceClientMapper().map(codeModel);
-                builder.serviceClient(serviceClient);
+                if (serviceClient != null) {
+                    builder.serviceClient(serviceClient);
 
-                serviceClientsMap.put(serviceClient, codeModel);
+                    serviceClientsMap.put(serviceClient, codeModel);
+                }
             }
         }
 

packages/http-client-java/generator/http-client-generator-core/src/main/java/com/microsoft/typespec/http/client/generator/core/preprocessor/tranformer/Transformer.java

@@ -54,9 +54,16 @@ public class Transformer {
       markFlattenedSchemas(codeModel);
     }
     transformOperationGroups(codeModel.getOperationGroups(), codeModel);
+    if (codeModel.getGlobalParameters() != null) {
+      for (Parameter parameter : codeModel.getGlobalParameters()) {
+        if (parameter.getLanguage().getJava() == null) {
+          renameVariable(parameter);
+        }
+      }
+    }
     // multi-clients for TypeSpec
     if (codeModel.getClients() != null) {
-      transformClients(codeModel.getClients(), codeModel);
+      transformClients(codeModel.getClients());
     }
     return codeModel;
   }
@@ -108,7 +115,7 @@ public class Transformer {
     }
   }
 
-  private void transformClients(List<Client> clients, CodeModel codeModel) {
+  private void transformClients(List<Client> clients) {
     for (Client client : clients) {
       renameClient(client);
 
@@ -138,6 +145,14 @@ public class Transformer {
           }
         }
       }
+
+      if (client.getGlobalParameters() != null) {
+        for (Parameter parameter : client.getGlobalParameters()) {
+          if (parameter.getLanguage().getJava() == null) {
+            renameVariable(parameter);
+          }
+        }
+      }
     }
   }
 

packages/http-client-java/generator/http-client-generator/src/main/java/com/microsoft/typespec/http/client/generator/mapper/TypeSpecClientMapper.java

@@ -35,7 +35,10 @@ public class TypeSpecClientMapper extends ClientMapper {
         Map<ServiceClient, Client> serviceClientsMap = new LinkedHashMap<>();
         TypeSpecServiceClientMapper mapper = new TypeSpecServiceClientMapper();
         for (Client client : clients) {
-            serviceClientsMap.put(mapper.map(client, codeModel), client);
+            ServiceClient serviceClient = mapper.map(client, codeModel);
+            if (serviceClient != null) {
+                serviceClientsMap.put(serviceClient, client);
+            }
         }
         return serviceClientsMap;
     }

packages/http-client-java/generator/http-client-generator/src/main/java/com/microsoft/typespec/http/client/generator/mapper/TypeSpecServiceClientMapper.java

@@ -55,6 +55,11 @@ public class TypeSpecServiceClientMapper extends ServiceClientMapper {
                 .forEach(og -> methodGroupClients.add(Mappers.getMethodGroupMapper().map(og, properties)));
         builder.methodGroupClients(methodGroupClients);
 
+        if (proxy == null && CoreUtils.isNullOrEmpty(methodGroupClients)) {
+            // No operation in this client, and no operation group as well. Abort the processing.
+            return null;
+        }
+
         if (proxy == null) {
             proxy = methodGroupClients.iterator().next().getProxy();
         }