Don't require a label for Entity updates

[lognaturel]

ODK Collect version

v2024.3.0 beta 8

Problem description

Entity updates without an explicit label fail silently.

Steps to reproduce the problem

1. Remove the value for the label Entity property for a form that updates Entities such as this one
2. Make sure that there is the offline column on the entities sheet and that its value is set to yes
3. Fill out and finalize a submission
4. See that the local Entity update was not applied

Expected behavior

Labels should be optional for Entity updates. Just like with other properties, if no value is specified in the update, the existing value should be carried forward. Label is required at creation time so there should always be one at update time.

Other information

I think this likely came from a misunderstanding around #6453 Labels are required for Entity creation but optional for updates.