Should not PASS on empty input

Considering that kubeval is probably run mostly in CI pipelines, I think it is funny that kubeval exits with 0 when it gets an empty input.

Do you guys consider that it is more likely that you would intentionally pass empty input to kubeval, **OR** could that possibly be a sign of a previous error that there is no input to validate?

I know you can't change this outright for BC, but a flag for "--fail-if-empty" would be a welcome addition for CI environments.

Start by tracing kubeval's command-line input handling and how validation behaves when no input is supplied. Add the requested opt-in --fail-if-empty behavior while preserving the current default, and verify that empty input fails only when the flag is enabled.

Written by the indexing model from the issue text.