Fix slugify when str is empty/none (BugFix)

[pedro-avalos]

Description

Currently the slugify function assumes the _string argument is not empty/None. However, I saw this when trying to run some tests on hinyari. One of the device attributes was empty, but existed.

Traceback (most recent call last):
  File "/home/ubuntu/checkbox/providers/resource/bin/udev_resource.py", line 198, in <module>
    main()
  File "/home/ubuntu/checkbox/providers/resource/bin/udev_resource.py", line 194, in main
    dump_udev_db(udev)
  File "/home/ubuntu/checkbox/providers/resource/bin/udev_resource.py", line 76, in dump_udev_db
    value = getattr(device, attribute)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/checkbox_support/parsers/udevadm.py", line 933, in vendor_slug
    return slugify(self.vendor)
           ^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/checkbox_support/helpers/slugify.py", line 33, in slugify
    if _string[0].isdigit():

Resolved issues

Documentation

Tests

Added a unit test

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.63%. Comparing base (7faa421) to head (6c5f174).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1768   +/-   ##
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  Coverage   49.63%   49.63%           
=======================================
  Files         377      377           
  Lines       40628    40630    +2     
  Branches     6829     6830    +1     
=======================================
+ Hits        20166    20168    +2     
  Misses      19740    19740           
  Partials      722      722           

Flag	Coverage Δ
checkbox-support	62.01% <100.00%> (+0.01%)	⬆️

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[Hook25]

Uhm this is a bit dangerous, None and "" would expand to the same symbol that is not even "". The semantic here is not clear at all imo. What if we introduce a kwarg with a default value simmilar to dict.get for a default when the string is None? For "" I would expect "" to be the expansion, similarly to what str.capitalize does for example