R1:95662ae8f1a7

Don't attempt to test capabilities on incomplete handles

Summary:
As backstory: I accidentally added the subscriber `PHID-USER-abcd` to `T1` on this install by calling `maniphest.edit`. I intended to edit `T1` on my local install.

This edit is permitted for messy technical reasons, described in T13429. It's not valid, but it's hard to prevent.

The state we reach is also possible even if the edit is rejected (i.e., someone can go manually update the database).

Regardless of how we get into this state…