Do not create entities with blank labels

[grzesiek2010]

Closes #6518

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

There isn't much to discuss here. Instead of blocking the creation of empty labels, we now block blank labels as well.

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?

It should just fix the issue described in #6518. I don't think it can affect anything else.

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

No.

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

[dbemke]

Should label concat(${circumference} ${species}) (it's trees registration form) be treated as blank label? Both Collect and Central treat it as "missing label"
trees_registration(5).xlsx.txt
https://staging.getodk.cloud/#/projects/101

[lognaturel]

If the label expression is concat(${circumference}, ${species}) and neither field is given a value, I would expect the label to be entirely missing because the expression would evaluate to "". That would be different from concat(${circumference}, " ", ${species}) which would result in a label of " ".

[dbemke]

Tested with Success!

Verified on devices with Androids: 10 , 15 (by Wiktor)

Verified cases:

• issue Local entities with only a space in the label #6518 doesn't occur
• trying to create entities with null values
• updating an entity to empty values
• concat with " " in the entity label