[java] align collection property initialization with default value logic

[andrey-fomin]

This update ensures that collection properties in Java generators are not initialized if no explicit default value is defined.

Key improvements:

• Collection properties will now be null by default, aligning their behavior with other data types when no default value is specified.
• Previously, collections were always initialized with an empty value, leading to their inclusion in payloads even without a defined default.

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 || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in Git BASH)
  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*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[jpfinne]

@andrey-fomin Please have a look at #18735 (comment)

Initialization of collections is a complex topic.
Several PR were not merged. So how is your PR different?

[andrey-fomin]

thanks @jpfinne

Actually my problem can be solved with containerDefaultToNull=true
I'll close this PR