Fix an issue where Phriction moves to new locations would fail with a "content required" error

Summary:
Ref T12793. I'd like to understand exactly when we broke this, but this seems to be a minimal fix that shouldn't do anything surprising.

When you move document /a/ to /a/b/ and that path doesn't exist yet, the Content transaction currently fails because there's "no content". The content gets added later by the "move" transaction but this is implicit.

To make this work, just ignore the "missing field" error. This is a little roundabout but unlikely to break anything in weird ways.

Test Plan:

• Moved document /a/b/ to /a/b/c/.
• Before patch: error about missing content.
• After patch: move worked properly.

Reviewers: chad, amckinley

Reviewed By: amckinley

Maniphest Tasks: T12793

Differential Revision: https://secure.phabricator.com/D18069