Fix “merging” a single volume annotation

[fm3]

Merging multiple annotations is implemented by reduceLeft(mergeTwo) – so if only one goes in, it is returned unchanged.
This means, the version reset was not executed in this case, which lead to errors for volume annotations.

URL of deployed dev instance (used for testing):

• https://fixmergevolumes.webknossos.xyz

Steps to test:

• Create volume tasktype, project and single-instance task. request it, trace and finish
• As admin, click view in task list (not individual user instance), should see volume data

Issues:

• fixes Merging volume annotations by id fails for single id #5197

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• Updated (unreleased) changelog
• Needs tracingstore update after deployment
• Ready for review

[jstriebel]

LGTM 👍