[BUG] Empty-string report path passed to CLI causes a traceback

[webknjaz]

This is another side of what I reported in codecov/test-results-action#89. It seems like the action passes a -f '' to the CLI invocation which causes that crash.
Obviously, the action should improve the files: input normalization, however, I believe that its CLI counterpart could benefit from better error handling as well and print out a nicer explanation message instead of a stack trace.