Adds isNull to codegenX java core classes that represent schemas

[spacether]

Adds isNull to codegenModel codegenParameter codegenProperty codegenResponse
This is different than nullable: True
isNull should be set to true for schemas of type null like:

  schema:
    type: 'null'

Note: this is a v3.1 feature
Adding this property and these getIsNull setIsNull methods is a feature break out for #7651

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master, 5.1.x, 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[spacether]

Closing and re opening for travis