Handle empty config files

[annaerdi]

This PR fixes Issue #73.

In details, it includes the following:

• If there is any of the default attackmate.yml file existing, but it is empty, the default config variables are loaded.
• If there is a file provided with the --config flag, but it's empty, the default config variables are loaded.
• There are unit tests added to test the parse_config function in __main__.py.
• Deprecated method warnings of the tests were fixed: the parse_obj function uses model_validate; and the warn function uses warning under the hood, so these are just beauty fixes.

[whotwagner]

Many yaml-files start with "###". So there are "empty" yaml-files out there that only have "###" in the first line

[annaerdi]

Updates:

• Yaml files containing "###" only are also handled as effectively empty files now
• Another issue I realised was, that if there are more than one of the default attackmate.yml files present, but the first one is empty, it didn't check the others, so that's also fixed now
• Added unit tests for covering both of the scenarios