PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

1. Fixing handling of serialization and deserialization with nested map and list return types.
2. Fixing bug where query is missing in the get call in the api_client.mustache.
3. Handling this issue [BUG][R] Endpoints returning of type Dictionary<K,V> are not properly represented in the "returnType" key in the output of generating -DdebugOperations #2201

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

@wing328 I have updated the serialization and deserialization handling all the complex and nested return types having maps and lists.
For the function toJSONString(), i updated the logic to construct the json, by checking if the value is null and then only adding to the json if its not null.