Java, adds checker framework

[spacether]

Java

• adds and uses the checker framework
• NullnessChecker added
• Fixed errors indicated by NullnessChecker
• Covariance used for collection (List/Map) inputs because those inputs are read from but not mutated

Some difficulties with Java Typing

• If type is List<@Nullable List<@Nullable Object>>
• then all allowed types should actually be the below ones because java treats Nullable T and T as separate classes, where T (NonNull) inherits from Nullable T
• List<@Nullable List<@Nullable Object>>
• List<@Nullable List<Object>>>
• List<List<@Nullable Object>>
• List<List<Object>>>
• the covariant type List<? extends @Nullable List<? extends @Nullable Object>> is used to cover all 4 cases above

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:

  mvn 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/python*.
  For Windows users, please run the script in Git BASH.