Improve performance of _get_zipimporters

[correctmost]

Description

_get_zipimporters can call isinstance millions of times when running pylint's import-error checker on a codebase like yt-dlp.

Checking for None first avoids the overhead of invoking isinstance.

Closes pylint-dev/pylint#9607.

Performance

I ran pylint with just the import-error checker on the yt-dlp codebase. With pylint 3.1.0 and astroid 3.1.0, linting takes ~38.2 seconds. With the fix applied, linting takes ~36.3 seconds.

Type of Changes

	Type
✓	🔨 Refactoring

[Pierre-Sassoulas]

Great find !

[Pierre-Sassoulas]

(pypy 3.8 and 3.10 are flaky, nothing to do on your end)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.59%. Comparing base (0ccc2e2) to head (5cd184f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2428      +/-   ##
==========================================
- Coverage   92.78%   92.59%   -0.19%     
==========================================
  Files          94       94              
  Lines       11098    11098              
==========================================
- Hits        10297    10276      -21     
- Misses        801      822      +21     

Flag	Coverage Δ
linux	92.59% <100.00%> (ø)
pypy	?
windows	?

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

Files	Coverage Δ
astroid/interpreter/_import/spec.py	97.47% <100.00%> (ø)

... and 5 files with indirect coverage changes