Fix cached_file in offline mode for cached non-existing files
# What does this PR do?

In offline mode, we sometimes returned the `_CACHED_NO_EXIST` constant instead of `None` for non-existing objects, since we now cache that non-existence. This PR fixes it.

Fixes #19186 
Thank you for such a quick fix!<|||||>_The documentation is not available anymore as the PR was closed or merged._