Better label color validation, fixes #7454

##### What does this MR do?

This PR improves the label color validation. Color isn't allowed to be empty. Additional it adds testcases for invalid input.

##### Why was this MR needed?

500 error is raised if color is empty.

##### What are the relevant issue numbers?

This PR fixes #7454

@randx Ok. Then I'll add the validation again, add the JS. I'll ping you in the evening when it's done. I'm not that productive on GitLab when I'm in work :smile: