Don't require a label for Entity updates

[grzesiek2010]

Closes #6511

Why is this the best possible solution? Were any other approaches considered?

There’s nothing to discuss here, as no alternative solutions were considered.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Creating entities without labels should still not be possible, as before. However, when updating existing entities, if a label is missing or empty, the existing label should be retained.

Do we need any specific form for testing your changes? If so, please attach one.

I used:
entities_update.xlsx
entities_registration.xlsx

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No.

Before submitting this PR, please make sure you have:

• added or modified tests for any new or changed behavior
• run ./gradlew connectedAndroidTest (or ./gradlew testLab) and confirmed all checks still pass
• added a comment above any new strings describing it for translators
• added any new strings with date formatting to DateFormatsTest
• verified that any code or assets from external sources are properly credited in comments and/or in the about file.
• verified that any new UI elements use theme colors. UI Components Style guidelines