Fix System.Text.Json formatter's support for named arguments object

Fixes #1028

src/StreamJsonRpc/SystemTextJsonFormatter.cs

@@ -487,9 +487,8 @@ public class SystemTextJsonFormatter : FormatterBase, IJsonRpcMessageFormatter,
             using (this.formatter.TrackDeserialization(this, parameters))
             {
                 // Support for opt-in to deserializing all named arguments into a single parameter.
-                if (parameters.Length == 1 && this.formatter.ApplicableMethodAttributeOnDeserializingMethod?.UseSingleObjectParameterDeserialization is true)
+                if (parameters.Length == 1 && this.formatter.ApplicableMethodAttributeOnDeserializingMethod?.UseSingleObjectParameterDeserialization is true && this.JsonArguments is not null)
                 {
-                    Assumes.NotNull(this.JsonArguments);
                     typedArguments[0] = this.JsonArguments.Value.Deserialize(parameters[0].ParameterType, this.formatter.massagedUserDataSerializerOptions);
                     return ArgumentMatchResult.Success;
                 }

test/StreamJsonRpc.Tests/JsonRpcTests.cs

@@ -1453,6 +1453,13 @@ public abstract class JsonRpcTests : TestBase
         Assert.Equal(7, sum);
     }
 
+    [Fact]
+    public async Task InvokeWithSingleObjectParameter_SupplyNoArgument()
+    {
+        int sum = await this.clientRpc.InvokeWithParameterObjectAsync<int>(nameof(Server.MethodWithSingleObjectParameterWithDefaultValue), cancellationToken: this.TimeoutToken);
+        Assert.Equal(-1, sum);
+    }
+
     [Fact]
     public async Task InvokeWithSingleObjectParameter_SendingExpectedObjectAndCancellationToken_InterfaceMethodAttributed()
     {
@@ -3337,6 +3344,12 @@ public abstract class JsonRpcTests : TestBase
             return fields.x + fields.y;
         }
 
+        [JsonRpcMethod(UseSingleObjectParameterDeserialization = true)]
+        public static int MethodWithSingleObjectParameterWithDefaultValue(XAndYProperties? arg = null)
+        {
+            return arg is not null ? arg.x + arg.y : -1;
+        }
+
         [JsonRpcMethod("test/MethodWithSingleObjectParameterWithProgress", UseSingleObjectParameterDeserialization = true)]
         public static int MethodWithSingleObjectParameterWithProgress(XAndYPropertiesWithProgress fields)
         {