`ignore` option's CLI default value includes repo root

[2bndy5]

I just noticed another problem when running the action as a python package. If the --ignore option is not specified then the default value is '' which gets translated as the repo root folder ./. Meaning, no files are linted when ignore is not specified to the cpp-linter executable script.

I'm opening this to add the issue to my backlog of needed changes.

[shenxianpeng]

Wow, a special test case 👍 looks like we also need to add test cases to cover more combinations.

[2bndy5]

Yeah I'm putting together some unit tests that can be run in the CI on future contributions. I've also ran the entire pkg through mypy for type checking; this is also now getting checked in the CI.

I'm currently trying to tackle #74 in a backward compatible way... Sorry I've been a bit distant on this project lately.

[shenxianpeng]

I like to have unit test (especially pytest) and mypy for type checking in CI. Enjoy and take your time!