[jazzy] Check for nullptrs when pushing new messages to the message cache (backport #2219)

[mergify]

Description

Adds a simple check to the message cache circular buffer when pushing nullptr messages.
Changed CircularMessageCache to report when dropping messages (null or too large for buffer).

• Fixes Lack of Null Pointer Checks in MessageCacheCircularBuffer::push Causes Crash #2115

Is this user-facing behavior change?

As it only adds a check if the message is a nullptr, this should not change any normal runtime behavior as this should always trigger a segmentation fault or at minimum UB.

Did you use Generative AI?

No

Additional Information

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This is an automatic backport of pull request #2219 done by [Mergify](https://mergify.com).

[mergify]

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Changes to be committed:
	modified:   rosbag2_cpp/include/rosbag2_cpp/cache/message_cache_circular_buffer.hpp
	modified:   rosbag2_cpp/src/rosbag2_cpp/cache/message_cache_circular_buffer.cpp
	modified:   rosbag2_cpp/test/rosbag2_cpp/test_circular_message_cache.cpp

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   rosbag2_cpp/include/rosbag2_cpp/cache/circular_message_cache.hpp
	both modified:   rosbag2_cpp/src/rosbag2_cpp/cache/circular_message_cache.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