ApiEditPage: Avoid PHP warning

ApiEditPage: Avoid PHP warning

In cases where $result is non-null but doesn't have the 'spam' key,
constructing the fallback array was raising a PHP warning.

Thinking it over, we should just use a switch instead of making an array
of known fallback messages. It's longer source code, but probably less
real work than constructing an array where only one entry will actually
be used.

Bug: T153590
Change-Id: I50349c56692c4b2f9a8e560a849c36db1082120e