Fix iterating over a DummyLogger when fast_dev_run > 0

[awaelchli]

What does this PR do?

Fixes #10201

The user had multiple loggers, i.e., Trainer(logger=[logger1, logger2]).
The user did this in training_step:

for logger in self.logger
    logger.xyz()

However, when Trainer(fast_dev_run=True, logger=[logger1, logger2]), self.logger is replaced with a DummyLogger and iterating over self.logger would result in infinite iteration.

The proposed fix makes the DummyLogger an empty iterable.
Test fails without the fix.

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

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