Previously null edits were not properly marked in EditResult objects
provided by the PageUpdater. This change should fix it, by ensuring
the original revision ID is always set on the EditResultBuilder if
the edit is a null edit.
I've also added some code to test this, so we hopefully don't
encounter an issue like this in the future. :)
Bug: T257766
Change-Id: I04bb058c64483967617958d86aa40a67c31071cb