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 agree. But removing html5 validation is not the best choice too because: 

* chrome & firefox supports it ( http://caniuse.com/form-validatio n)
* html5 validation will be available in future version of safari ( I hope:) )

So I propose you to proceed with your solution: 

> save button is disabled until you enter something