Lint: Fix features module

[ByteOtter]

What this PR does / why we need it:

This PR fixes various typing and some smaller logic issues within the features module.
Fixes include:

• Raising a RuntimeError before calling any filter function if flavor is None to avoid passing None values
• Fixing a call to an undefined variable in a ValueError in the Parser class
• Removing unnecessary Optional type hints in cases where default values are set
• Explicitly suppressing the return value of functions to mark intent
• Fixing the syntax of Callable in function signatures
• Explicitly casting to a networkx.DiGraph in the _exclude_from_filter_set function to avoid accessing successors of Unknown type

While doing these fixes, I noticed that many values are initialized as None and then treated like a regular str type in the code that follows. I tried to assume the most logical approach to determine if these values are allowed to be None in their respective context.

Todo:

• CName class
• CName Main module
• Noneable flavor variable in __main__.py without proper handling

Which issue(s) this PR fixes:
Tracks #152

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.36%. Comparing base (5126dc7) to head (bf24225).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gardenlinux/features/__main__.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #218      +/-   ##
==========================================
- Coverage   92.37%   92.36%   -0.01%     
==========================================
  Files          40       40              
  Lines        1822     1821       -1     
==========================================
- Hits         1683     1682       -1     
  Misses        139      139              

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[yeoldegrove]

Changes look ok to me. Good to merge if the linting errors are fixed.

[ByteOtter]

Changes look ok to me. Good to merge if the linting errors are fixed.

The current CI failure is a result of errors in other modules. I'm going module by module, next one is flavors.