Fix last.ckpt only being saved when another checkpoint has been created

[SkafteNicki]

What does this PR do?

Fixes #20245

Our documentation says about save_last in ModelCheckpoint:

save_last: When True, saves a last.ckpt copy whenever a checkpoint file gets saved. Can be set to 'link' on a local filesystem to create a symbolic link. This allows accessing the latest checkpoint a deterministic manner. Default: None.

meaning that last.ckpt should not be created unless another checkpoint already exists. However, in specific situations (see linked issue) currently it may happen that last.ckpt is saved before any other checkpoint. This PR fixes that.g

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📚 Documentation preview 📚: https://pytorch-lightning--21244.org.readthedocs.build/en/21244/