Handle the absence of a fresh eval checkpoint in `run_local`.

It is ~unexpected condition for an eval checkpoint to not be available after a train call to the estimator.  There is a corner case when it is possible, but that's going to be resolved soon.

This case is handled for continuous (distributed) evaluation differently.  Instead of erroring out, we skip evaluation runs.  That behavior is captured in the `test_skip_evaluation_due_to_ckpt` test.

PiperOrigin-RevId: 170919925