Error marker on annotations with required multi-value features is missing

[reckart]

Describe the bug
When a multi-value string feature is mandatory, but the feature is not filled in, a error marker should be rendered - just as with mandatory string features.

To Reproduce
Steps to reproduce the behavior:

1. Create project and a layer having a multi-value string feature
2. Make the multi-value string feature feature required in the feature settings
3. Create an annotation
4. See that the annotation does not bear an error marker

Expected behavior
In the brat editor, the annotation should have a red glow.

Please complete the following information:

• Version and build ID: 30.4

Additional context
Add any other context about the problem here.