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

Ok. The reason was when saving the the label without a text, the save button gets disabled, and no error message pops up... I could a the `js-disable-button` to the save button (Same like in the notes form, where the save button is disabled until you enter something)