Fix updating boards through API

What does this MR do?

There were a couple of issues here:

1. Because an empty labels key was in the params, it was blowing up as ActiveRecord's expecting a board's labels to be an association.
2. The API wouldn't allow removing labels, assignee, etc.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/11440.