Add defaultToEmptyContainer option to java generators

[wing328]

to close #21505

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• Read the contribution guidelines.
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Summary by cubic

Adds a defaultToEmptyContainer option to Java-based generators. When enabled, container fields default to empty collections instead of null.

• New Features

  • Added CLI/config option defaultToEmptyContainer in AbstractJavaCodegen and updated docs across Java and Groovy generators.
  • Reduces null checks in generated code and makes iteration/collection operations safer.

• Migration

  • Set additionalProperties defaultToEmptyContainer=true (or config option) when generating code.

^{Written for commit 7d73a4c. Summary will update on new commits.}

[cubic-dev-ai]

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