Don't emit empty "nlohmann" namespace in C++. Fixes #504

src/ConvenienceRenderer.ts

@@ -331,6 +331,10 @@ export abstract class ConvenienceRenderer extends Renderer {
         return !this._namedUnions.isEmpty();
     }
 
+    protected get haveNamedTypes(): boolean {
+        return !this._namedTypes.isEmpty();
+    }
+
     protected get haveUnions(): boolean {
         return this._haveUnions;
     }

src/Language/CPlusPlus.ts

@@ -754,6 +754,7 @@ inline ${optionalType}<T> get_optional(const json &j, const char *property) {
             this.emitNamespace(this._namespaceName, this.emitTypes);
         }
         if (this._justTypes) return;
+        if (!this.haveNamedTypes) return;
 
         this.ensureBlankLine();
         this.emitNamespace("nlohmann", () => {