Message: Correct output of wfMessage( 'non-existent-msg' )->text()

The output of Message::text() should always be acceptable to pass into
external html escaping, such as when the response is returned over an
API request and escaped by the client side code. Calling ->text() on a
non-existent key was returning the entity encoded value which leads to
double encoding down the line, this patch fixes that oversight.

Bug: 66199
Change-Id: Ieec94d4e4c7e5c36e5e68bbf01792e96368e54e0