Fixes #33449 Snapping empty layer when adding new feature

[troopa81]

Description

If the layer is empty and we create a new features, these ones are not indexed. This PR reset the mIsEmptyLayer boolean when a feature is added so the index could be build.

Checklist

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include Fixes #11111 at the bottom of the commit message
• I have read the QGIS Coding Standards and this PR complies with them
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit
• I have evaluated whether it is appropriate for this PR to be backported, backport requests are left as label or comment

[nyalldawson]

Unrelated test failure