Adds hasVars

[spacether]

Adds hasVars to java Schema classes
This is needed if one is generating classes for object schemas and one only wants to generate the class if properties exists.
One could almost do this with #vars but that doesn't work because using #vars mean we move into the context of iterating over each var when we just need to know if there are vars.

hasVars is tested in the below models:

• CodegenModel
• CodegenProperty
• CodegenParameter
• CodegenResponse

This will be used in:
#8325

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

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[spacether]

Circle ci error with dart is odd. My code should not cause it. Rebasing and regenerating samples to see if that fixes it.

[spacether]

Seeing this same issue here: #8877
It looks like master branch dart tests are failing in circleci.

[spacether]

Dart error is unrelated to this PR