[23929] Allow to send empty payloads in dispose/unregister ops (backport #6217)

[mergify]

Description

This pull request improves the handling of dispose and unregister operations when payloads are empty, and enhances test coverage for these scenarios. Until now, it was not possible to send those messages with empty payloads, now it is allowed as long as a valid instance handle is provided.

• Update DataWriterImpl to be able to write Unregister/Dispose messages with empty payloads.
• Add tests for the case above
• Update RTPSStatelessReader to be able to read Unregister/Dispose messages with empty payloads.
• Add tests for the case above
• Update RTPSStatefulReader to be able to read Unregister/Dispose messages with empty payloads.
• Add tests for the case above
• Modified topic_instances example to correctly print the color name even when no payload is passed.

@Mergifyio backport 3.4.x 3.3.x 3.2.x 2.14.x

Contributor Checklist

• Commit messages follow the project guidelines.

• The code follows the style guidelines of this project.

• Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally

• N/A: Any new/modified methods have been properly documented using Doxygen.

• N/A: Any new configuration API has an equivalent XML API (with the corresponding XSD extension)

• NO: Changes are backport compatible: they do NOT break ABI nor change library core behavior.

• Changes are API compatible.

• N/A: New feature has been added to the versions.md file (if applicable).

• N/A: New feature has been documented/Current behavior is correctly described in the documentation.

• Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• N/A: If this is a critical bug fix, backports to the critical-only supported branches have been requested.
• Check CI results: changes do not issue any warning.
• Check CI results: failing tests are unrelated with the changes.

---

This is an automatic backport of pull request #6217 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 47bfac0 has failed:

On branch mergify/bp/3.3.x/pr-6217
Your branch is up to date with 'origin/3.3.x'.

You are currently cherry-picking commit 47bfac03.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   examples/cpp/topic_instances/SubscriberApp.cpp
	modified:   examples/cpp/topic_instances/SubscriberApp.hpp
	modified:   src/cpp/fastdds/publisher/DataWriterImpl.cpp
	modified:   src/cpp/rtps/reader/StatefulReader.cpp
	modified:   src/cpp/rtps/reader/StatelessReader.cpp
	modified:   test/unittest/rtps/reader/CMakeLists.txt
	modified:   test/unittest/rtps/reader/StatefulReaderTests.cpp
	new file:   test/unittest/rtps/reader/StatelessReaderTests.cpp

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   test/unittest/dds/publisher/DataWriterTests.cpp

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[mergify]

🧪 CI Insights

Here's what we observed from your CI run for 7a82ff6.

❌ Job Failures

Pipeline	Job	Health on 3.3.x	Retries	🔍 CI Insights	📄 Logs
Fast DDS MacOS CI	mac-ci / fastdds_test ()		0	View	View
Fast DDS Windows CI	windows-ci / fastdds_test (RelWithDebInfo, examples), v142		0	View	View