Fix empty docstring

[ahartikainen]

Fixes #1177

Description

Fix import when docstrings are missing.

Checklist

• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[codecov]

Codecov Report

Merging #1192 into master will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #1192      +/-   ##
==========================================
- Coverage   93.26%   93.25%   -0.01%     
==========================================
  Files          94       94              
  Lines        9381     9383       +2     
==========================================
+ Hits         8749     8750       +1     
- Misses        632      633       +1     

Impacted Files	Coverage Δ
arviz/plots/backends/__init__.py	29.62% <50.00%> (+0.51%)	⬆️

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