Fix Maniphest fatal when editing a task without changing anything

Summary:
Transaction editor attempted to do things with an empty transaction array, just
skip editing ops if nothing changed.

Test Plan:
Edited a Maniphest task without changing anything.

Reviewed By: tuomaspelkonen
Reviewers: tuomaspelkonen, jungejason, aran
CC: sandra, anjali, aran, tuomaspelkonen
Differential Revision: 248