chore: exit evaluation script if nothing to do (#1910)
Relates to CI ingest-tests. The last step of test-ingest.sh is to
calculate evaluation metrics (comparing gold set standard outputs with
actual output files). If no output files were created, as *should* be
the case right now in CI for all python versions other than 3.10 (that
only test a limited number of
files/connectors),`unstructured/ingest/evaluate.py` would fail.
