Fix #668: Safely handle empty custom_atlases.conf file

[kalpana-pardeep-rassani]

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Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?
The current implementation crashes with a KeyError: 'atlases' if the local custom_atlases.conf file exists but is empty (contains no sections). This prevents the brainglobe list command from running.

What does this PR do?
This PR modifies the get_all_atlases_lastversions function in brainglobe_atlasapi/list_atlases.py to use a safe .get("atlases", {}) method when reading the custom atlas configuration. This ensures that an empty dictionary is used if the 'atlases' section is missing, thus preventing the crash.

References

Fixes #668

How has this PR been tested?

The fix was tested locally by ensuring that the brainglobe list command runs successfully without crashing when the ~/.brainglobe/custom_atlases.conf file is present but completely empty.

Is this a breaking change?

No. This is a fix for an existing bug and does not change any public API or required functionality.

Does this PR require an update to the documentation?

No.

Checklist:

• [done] The code has been tested locally
• [done] Tests have been added to cover all new functionality (unit & integration)
• The documentation has been updated to reflect any changes
• [done] The code has been formatted with pre-commit
  Note on Test Failures:

All code changes related to Issue #668 (empty config file) have been verified locally. When running the full test suite, I encountered 2 ConnectionErrors (due to the GIN server being unreachable) and 1 FileNotFoundError (due to missing local test data), which are external to this fix. The patch itself is clean.