fix: add default to path_2_processed arg

[balajtimate]

Description

Add None to path_2_processed: Optional[Path] as default, to prevent the validation error by Pydantic when initializing the Args class. This caused an issue during the publish docker image step of the CI.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

Please carefully read these items and tick them off if the statements are true
or do not apply.

• I have performed a self-review of my own code
• My code follows the existing coding style, lints and generates no new
  warnings
• I have added type annotations to all function/method signatures, and I
  have added type annotations for any local variables that are non-trivial,
  potentially ambiguous or might otherwise benefit from explicit typing.
• I have commented my code in hard-to-understand areas
• I have added ["Google-style docstrings"] to all new modules, classes,
  methods/functions or updated previously existing ones
• I have added tests that prove my fix is effective or that my feature
  works
• New and existing unit tests pass locally with my changes and I have not
  reduced the code coverage relative to the previous state
• I have updated any sections of the app's documentation that are affected
  by the proposed changes

If for some reason you are unable to tick off all boxes, please leave a
comment explaining the issue you are facing so that we can work on it
together.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (58d7bbf) 99.80% compared to head (32e11f4) 99.80%.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #137   +/-   ##
=======================================
  Coverage   99.80%   99.80%           
=======================================
  Files          12       12           
  Lines        1039     1039           
=======================================
  Hits         1037     1037           
  Misses          2        2           

Files Changed	Coverage Δ
htsinfer/models.py	100.00% <100.00%> (ø)

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