Save empty fields as no row, not an empty row

Summary: When a user stores the empty string in an auxiliary field, simply don't
store it, and delete it if it already exists.

Test Plan: Edited a revision with an empty "Quack" field, got an empty row in
the DB. Applied patch, edited empty again, row went away. Edited empty again,
still no row. Edited and put something in the field, got a row.

Reviewers: jungejason, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 865