fix issue in labeller

[OriolAbril]

Description

When combining with NoVarLabeller or NoModelLabeller there were some stray
nones being printed. This should fix the issue.

Checklist

• Does the PR follow official PR format?
• Is the code style correct (follows pylint and black guidelines)?
• Is the fix listed in the Maintenance and fixes section of the changelog?

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📚 Documentation preview 📚: https://arviz--2200.org.readthedocs.build/en/2200/

[codecov]

Codecov Report

Merging #2200 (b06f4a5) into main (4fc5000) will decrease coverage by 0.01%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##             main    #2200      +/-   ##
==========================================
- Coverage   89.91%   89.91%   -0.01%     
==========================================
  Files         119      119              
  Lines       12419    12421       +2     
==========================================
+ Hits        11167    11168       +1     
- Misses       1252     1253       +1     

Impacted Files	Coverage Δ
arviz/labels.py	93.50% <80.00%> (-1.17%)	⬇️

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