Introduce third "not-null" option for board filters

Problem to solve

When filtering by board, there should be an option for "any not-null value" that only returns issues with a value in that category (e.g. an assignee, label, milestone or weight).

Intended users

Anyone who uses a filter on the issue boards.

Further details

On the backend, there is already support for such an option. If it's decided that this option will not be introduced, we should refactor the backend to remove "All")

Proposal

What does success look like, and how can we measure that?

The user can easily understand the difference between "any (not-null value)" and "any (not-null AND null)" value, and could be able to choose between the two. This experience should be consistent.