[BUG][java][restclient] Fix NPE when query param with value null is exploded

[jorgerod]

Closes #23504

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Summary by cubic

Fixes a NullPointerException in the Java restclient generator when exploded query parameters are null. Generated APIs now guard exploded object/array query params with a null check for form and deepObject styles.

• Bug Fixes
  • In Java/libraries/restclient/api.mustache, wrap exploded param mapping with if (param != null) { ... } when isExplode and the param has fields.
  • Updated sample QueryApi.java to add the same null guards for exploded objects and arrays.
  • Added a test to ensure JavaClientCodegen.RESTCLIENT generates the null check for exploded query params.

^{Written for commit 8f927df. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files