[Fix] Handle Missing Contents in Messages in MessageTokenLimiter

[WaelKarkoub]

Why are these changes needed?

Fixes a bug for cases when contents are missing from a message. Added two tests:

1. Tests the behavior of the transform when the content type is unexpected.
2. Check if the transform always returns a message

Related issue number

Closes #2340

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[gagb]

Thank you! Nice catch