Ref T13614. When an omnipotent user calls "synchronizeWorkingCopyBeforeWrite()", we record a WorkingCopyVersion record with a null "userPHID". The UI then renders this as "Unknown Object (????)".

Improve this behavior:

• When no PHID is available, just render nothing in the UI (this doesn't seem meaningfully different from no version existing at all).
• Allow callers to provide an acting user PHID, similar to Editor.

There's currently no way to perform this kind of write legitimately in the upstream, but T13614 is providing one.

• Wrote a script that calls "synchronizeWorkingCopyBeforeWrite()" as the omnipotent user.
• Ran script, saw "Unknown Object (????)" in the UI.
• Applied UI fix, saw empty UI.
• Applied "acting as" fix, modified script to act as the Diffusion application, ran script, saw "Diffusion" attribution in UI.