Fix the case where logger=None is passed to Trainer

[daniellepintz]

What does this PR do?

Fixes a small bug introduced in #11920 for the case where logger=None is passed to the Trainer.

Before this PR, when logger=None is passed to the Trainer, we see this error:

    @property
    def log_dir(self) -> Optional[str]:
        if len(self.loggers) == 1:
            if isinstance(self.logger, TensorBoardLogger):
                dirpath = self.logger.log_dir
            else:
>               dirpath = self.logger.save_dir
E               AttributeError: 'NoneType' object has no attribute 'save_dir'

full output: https://gist.github.com/daniellepintz/f603473b82c1f21d8299eaae56b33001

This is because trainer.loggers is being set to [None] here: https://github.com/PyTorchLightning/pytorch-lightning/blob/5df519572f78ae5e23b0aab18fcbda06df7ff8dd/pytorch_lightning/trainer/connectors/logger_connector/logger_connector.py#L101
so in the log_dir property it thinks len(self.loggers) == 1.

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

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[krshrimali]

LGTM, thanks for fixing this. ⚡ Before we forget about this, can you also create an issue from this comment?

[Borda]

add chlog

[daniellepintz]

@carmocca @tchaton mind taking a look? Your review is needed to merge

[daniellepintz]

Ready to be merged!