[train] update config dataclass repr to check against None

[matthewdeng]

Why are these changes needed?

This is needed because custom pyarrow filesystems will raise a SIGSEGV if checking equality against None.

Context: https://ray-distributed.slack.com/archives/CSX7HVB5L/p1698796987044039

Related issue number

apache/arrow#38535

Checks

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• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[justinvyu]

Thanks!