Improve performance of _get_zipimporters

_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.

Author: correctmost

astroid/interpreter/_import/spec.py

@@ -341,7 +341,7 @@ def _is_setuptools_namespace(location: pathlib.Path) -> bool:
 
 def _get_zipimporters() -> Iterator[tuple[str, zipimport.zipimporter]]:
     for filepath, importer in sys.path_importer_cache.items():
-        if isinstance(importer, zipimport.zipimporter):
+        if importer is not None and isinstance(importer, zipimport.zipimporter):
             yield filepath, importer