Fix issues when RichProgressBar disabled

[awaelchli]

What does this PR do?

Fixes issues found in #15321. When disabling the progress bar and using it in the Trainer, we get assertion errors and NoneType attribute errors. The added test reproduces several issues.

Q: When do we have a disabled progress bar in the trainer?
A: When running with DDP, we internally call progress_bar.disable() on rank > 0 to avoid showing a bar for each process.

This PR fixes RichProgressBar in combination with DDP and other multi-process strategies.

Does your PR introduce any breaking changes? If yes, please list them.

No.

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cc @Borda @kaushikb11 @rohitgr7