Avoid "Action with no effect" for auto-claim statuses after manually removing assignee

Summary:
Sometime you just want to flag something as Resolved and keep that task claimed by nobody.

But, there are some task statuses that can auto-claim, and "Resolved" is one of these.
So, if you "Resolved", Phorge tries to set yourself as claimer.

Keeping that "claimed by nobody" is a bit tricky and also generates a confusing warning.

In fact, after you "Resolved", you can override the defaults with:

• Add Action > Assign / Claim > (nobody)

The problem is, on saving, the above action causes this warning:

Action With No Effect
One of your actions has no effect:
The task already has the selected owner.
Apply remaining actions?
[ Cancel ] [ Apply Remaining Actions ]

That warning "The task already has the selected owner" really means
"The task is already claimed by nobody" and, indeed, that is exactly what the user wants.

This patch intercepts the above action, and prevents the related confusing "non-effect" warning.

Thanks to hard troubleshooting from user https://we.phorge.it/p/aklapper/

See also https://we.phorge.it/D25476

Closes T15164

Test Plan:
Task 1 open unassigned:

1. Change Status to Resolved
2. Preview yourself as Claimer
3. Add Action > Assign / Claim, and set <nothing>
4. Save and, instead of any confusing warning, only the Status changes.

Task 2 open unassigned:

1. Change Status to Resolved
2. Preview yourself as Claimer
3. Add Action > Assign / Claim, keep it as-is
4. Save and, it works as expected (just like before)

Task 3 open unassigned:

1. Change Status to Resolved
2. Preview yourself as Claimer
3. Add Action > Assign / Claim, set to somebody else
4. Save and, it works as expected (just like before)

Reviewers: O1 Blessed Committers, aklapper

Reviewed By: O1 Blessed Committers, aklapper

Subscribers: tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15164

Differential Revision: https://we.phorge.it/D25550