chore: make RESTObject._short_print_attrs always present

[JohnVillalovos]

Always create RESTObject._short_print_attrs with a default value of
None.

This way we don't need to use hasattr() and we will know the type of
the attribute.

[codecov-commenter]

Codecov Report

Merging #1410 (6d55120) into master (4f79dff) will not change coverage.
The diff coverage is 83.33%.

@@           Coverage Diff           @@
##           master    #1410   +/-   ##
=======================================
  Coverage   80.20%   80.20%           
=======================================
  Files          73       73           
  Lines        4067     4067           
=======================================
  Hits         3262     3262           
  Misses        805      805           

Flag	Coverage Δ
unit	80.20% <83.33%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
gitlab/v4/cli.py	41.63% <0.00%> (ø)
gitlab/base.py	88.60% <100.00%> (+0.07%)	⬆️
gitlab/client.py	78.96% <100.00%> (-0.06%)	⬇️

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[max-wittig]

Makes sense