fix bug 47150 (sfEditFormPreloadText no longer works as it used to)

Special:FormEdit explicitly turned preloading off if it was unset by the request.
It now leaves the setting alone.

Change-Id: I080bd4473635c26e8a8b853d5272f892ed00b639
Merge "fix bug 47150 (sfEditFormPreloadText no longer works as it used to)"