Small bug fix for default value of command line flag `eval_split` passed to `train.py`.
Small bug fix for default value of command line flag `eval_split` passed to `train.py`.

The empty string throws an `absl.flags._exceptions.IllegalFlagValueError` for the `enum` flag, but a `None` default value is permitted.

PiperOrigin-RevId: 305480732

google-research/meta-dataset