fix(bzlmod): do not fail pip.parse with empty requirements.txt

[aignas]

With this change we handle correctly the case where the requirements.txt
might be empty (e.g. during onboarding to rules_python). I have added
an integration test because this usecase seems important enough to be covered
to ensure that we have no regressions. It also creates a place to add more
pip.parse and pip_parse tests.

Closes #1623.

[rickeylev]

just some small nits, otherwise lgtm