fix: write undefined JSON RPC input body (#2075)

Author: kstich

codegen/smithy-aws-typescript-codegen/src/main/java/software/amazon/smithy/aws/typescript/codegen/JsonRpcProtocolGenerator.java

@@ -107,6 +107,14 @@ private DocumentMemberSerVisitor getMemberSerVisitor(GenerationContext context,
         return new JsonMemberSerVisitor(context, dataSource, getDocumentTimestampFormat());
     }
 
+    @Override
+    protected boolean writeUndefinedInputBody(GenerationContext context, OperationShape operation) {
+        TypeScriptWriter writer = context.getWriter();
+
+        writer.write("const body = '{}';");
+        return true;
+    }
+
     @Override
     protected void writeErrorCodeParser(GenerationContext context) {
         TypeScriptWriter writer = context.getWriter();