convert empty structure payloads to null

Author: smilkuri

smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/integration/HttpBindingProtocolGenerator.java

@@ -2486,6 +2486,14 @@ private HttpBinding readPayload(
                         Location.PAYLOAD, "data", binding.getMember(), target));
                 }
             );
+        } else if (target instanceof StructureShape) {
+            writer.write("contents.$L = $L;", binding.getMemberName(), getOutputValue(context,
+                Location.PAYLOAD, "data", binding.getMember(), target));
+            // Handle empty objects for structure shapes - convert to null
+            writer.write("if (contents.$L && Object.keys(contents.$L).length === 0) {",
+                binding.getMemberName(), binding.getMemberName());
+            writer.write("  contents.$L = null;", binding.getMemberName());
+            writer.write("}");
         } else {
             writer.write("contents.$L = $L;", binding.getMemberName(), getOutputValue(context,
                 Location.PAYLOAD, "data", binding.getMember(), target));