chore: added fix nested enum, and keyword solution argument

[kridai]

Fixes

Made self._solution a keyword argument instead of positional argument.
For nestedResponseModel.handlebars added a check to only produce nested classes if properties are present. This is prevent empty classes generated in case of enum attributes

Checklist

• I acknowledge that all my contributions will be made under the project's license
• Run make test-docker
• Verify affected language according to the code change:
  • Generate twilio-java from our OpenAPI specification using the scripts/build_twilio_library.py using python scripts/build_twilio_library.py path/to/twilio-oai/spec/yaml path/to/twilio-java -l java and inspect the diff
  • Run make test in twilio-java
  • Create a pull request in twilio-java
  • Provide a link below to the pull request, this ensures that the generated code has been verified
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

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