Add support for multi-line allow-only and fail-on arguments

[anmol-aidora]

When setting the allow-only and fail-on arguments, the list can get very long very fast, especially when switching partial to false. There is a need to make sure multi-line arguments work properly.

Currently when passing multi-line allow-only and fail-on arguments in pyproject.toml, the fail_on_licenses and allow_only_licenses sets were found to contain the empty string ''.

e.g.

allow-only="""
MIT;Apache;
"""

sets allow_only_licenses to:

{'', 'Apache', 'MIT'}

The code checks for the same and removes it from the sets, such that there is no empty string:

{'Apache', 'MIT'}