[21733] Add default-less union struct with extra member

[juanlofer-eprosima]

Description

Type used for regression test, in which DynamicData unions fail to be deserialized when no member is selected: eProsima/Fast-DDS#5279

Contributor Checklist

• Commit messages follow the project guidelines.
• Any new/modified type has been properly reflected on test/feature/dynamic_types/dds_types_tests tests in a Pull Request.
• Any new/modified type has been properly updated on test/dds/xtypes tests using update_header_and_create_cases.py script in a Pull Request.
• N/A For any new IDL file, Fast DDS update_generated_code_from_idl.sh script adding a new line in file_needing_output_dir list in a Pull Request.

Reviewer Checklist

• N/A The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.