cache empty result for picklist label retrieval in _ODataClient

[pkontek]

This pull request updates the _optionset_map method to cache empty results instead of returning None when no items are found. This change ensures that empty lookups are cached, which can improve performance by avoiding repeated queries for missing option sets.

Caching improvements:

• Modified _optionset_map in src/PowerPlatform/Dataverse/data/_odata.py to cache empty results in _picklist_label_cache and return an empty dictionary instead of None when no items are found.

[Copilot]

Pull request overview

This PR optimizes the picklist label retrieval logic by caching empty results when no option sets are found, preventing redundant queries for non-existent data.

Key Changes:

• Modified _optionset_map to cache empty results instead of returning None
• Return empty dictionary {} instead of None for consistency

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[Copilot]

Changing the return type from None to {} is a breaking change for consumers who check for None (e.g., if result is None). This could cause existing code to behave incorrectly. Consider deprecating the None return in a separate release or updating all callers to handle empty dict, or document this breaking change explicitly in release notes.

[pkontek]

It’s the same value returned when the Attribute is not a picklist. In my opinion, this approach is more consistent.