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

@Razer6 what browser do you use? Safari 7.0 has problem with html5 validation