feat(uui-color-picker): allow the value to be empty

[loivsen]

Description

Allows the UUI Color picker to have an empty string as value. This shows up as fully transparent. Deleting a picked color now sets the value to empty string.

Fixed an issue where when typing in a color manually in the input field it would not update the value.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Chore (minor updates related to the tooling or maintenance of the repository, does not impact compiled assets)

Motivation and context

We dont want the color picker to automatically pick a color for us.