R1:add8333b98cb

Improve behavior of "owner" transaction in "maniphest.edit" endpoint

Summary:
Fixes T10117.

- I accidentally broke setting `null` to unassign tasks at some point when I added richer validation.
- Raise a better error if the user passes junk.

Test Plan:
- Unassigned a task via API and web UI.
- Reassigned a task via API and web UI.
- Tried to do an invalid assign via API, got a sensible error.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10117

Differential Revision: https://secure.