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

There is already a validation in the model: https://github.com/gitlabhq/gitlabhq/pull/7465/files#diff-75f7bf70aff64257ed14f4aadf1db50dL12

This validation then generates the error message which is shown on top of the page.