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.phabricator.com/D14992