planemo lint should spot empty *.loc files

I would like ``planemo lint`` to report empty location files with a warning - they should at least have comments describing the format of the entries. This applies to both ``*.loc`` and ``*.loc.sample``

See e.g. https://github.com/galaxyproject/tools-iuc/issues/2082 for repositories having empty ``*.loc.sample`` files

Start with the planemo lint entry point and trace how lint rules and warnings are organized. Add coverage for empty *.loc and *.loc.sample files, with comments accepted as non-empty content; done means lint reports a warning for empty files in both forms.

Written by the indexing model from the issue text.