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GH-41686: [Java] Nullability of struct child vectors not preserved in TransferPair (#41785)

### Rationale for this change

Nullability of the struct child vectors are not preserved in the StructWriter templates. 

### What changes are included in this PR?

- [X] Adding nullability and test cases. 
- [X] Updating to JUNIT5

### Are these changes tested?

New test case added and the change is being validated by existing tests. 

### Are there any user-facing changes?

No
* GitHub Issue: #41686

Authored-by: Vibhatha Lakmal Abeykoon <vibhatha@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>