Do not notify text watchers when replace is a no-op

Bug 6344997

The early exit we used to do when both replaced and replacement
strings were empty has been added back.

Only this time it correctly also makes sure no spans from the
replacement string would get added with a 0-length.

Change-Id: Ifc38a7e3619c57aa7647c0a8e63d7627d86f1036