Correct TextMessageCompressor's cache parameter initialising on import

[marklysze]

Why are these changes needed?

The TextMessageCompressor's cache parameter for __init__() has a default value of Cache.disk(). This is causing it to run on import rather than just on instantiation.

This PR changes the default to None and the Cache.disk() is executed within __init__.

Related issue number

Raised by @omarhurani, #63

Checks

• [ ] I've included any doc changes needed for https://autogen-ai.github.io/autogen/. See https://autogen-ai.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.
• [X] I've made sure all auto checks have passed.

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[omarhurani]

Tested it, no .cache dir is created when doing import autogen. Thanks!