updated result initialization

lib/codegen/java/async_http_client.dart

@@ -121,7 +121,7 @@ public class Main {
     String? boundary,
   }) {
     try {
-      String result = "";
+      String result = '';
       bool hasBody = false;
 
       var rec = getValidRequestUri(
@@ -134,6 +134,13 @@ public class Main {
         return "";
       }
 
+      result += kTemplateStart;
+      if (requestModel.hasBody && requestModel.hasFileInFormData) {
+        result += kTemplateMultipartImport;
+      }
+      result += kTemplateMainClassMainMethodStart;
+      result += kTemplateAsyncHttpClientTryBlockStart;
+
       var url = stripUriParams(uri);
 
       // contains the HTTP method associated with the request