[train] Do not raise warning when no results were reported

[krfricke]

Why are these changes needed?

When no result is reported, a training run will print this message at the end of training:

2023-09-07 18:53:20,003 WARNING experiment_analysis.py:205 -- Failed to fetch metrics for 1 trial(s):
- TorchTrainer_78aea_00000: JSONDecodeError('Expecting value: line 1 column 1 (char 0)')

This is due to the results json file being empty. Instead, we should identify empty files early and return an empty dataframe, avoiding the error message.

Related issue number

Closes #39430

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[justinvyu]

Thanks! Can we also do the same thing for the csv case?