fix: ignore None type categories

[desobolevsky]

Actual PR for #1325
Duplicate for #1416, but since this PR seems to be outdated and no longer update/rebased to the latest version, doing this.

[desobolevsky]

Hey, @fabclmnt! Don't wanna be intrusive, but could you please take a look at my PR? :)
The tests seem to fail for some external reasons not connected to changes at the code.
Thanks in advance!

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.22%. Comparing base (1314e0d) to head (c4055c7).
Report is 11 commits behind head on develop.

Files	Patch %	Lines
...g/report/structure/variables/render_categorical.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1632      +/-   ##
===========================================
+ Coverage    90.06%   90.22%   +0.16%     
===========================================
  Files          196      197       +1     
  Lines         6439     6453      +14     
===========================================
+ Hits          5799     5822      +23     
+ Misses         640      631       -9     

Flag	Coverage Δ
py3.8-ubuntu-22.04-pandas	90.22% <50.00%> (+0.16%)	⬆️

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[alexbarros]

sry for the delay @desobolevsky, I will be having a look at this week. My questions with this PR is more related to the root cause of the problem, which is still not clear. This workaround may lead to silent errors, since the user is expecting their unicode categories to be categorized as Other Symbol, just like 😀 and 🔥.

Also, when creating a fix for specific use case like this we expect the addition of a unit test tests/issues/test_issue1325.py to ensure the issue is fixed and we don't run into it again in the future.