parser: Simplify code for {{#int:}} if the message doesn't exist

parser: Simplify code for {{#int:}} if the message doesn't exist

Ever since 184658eb32f6, the output of a non-existing message will be
HTML safe, regardless of output format, so we can treat non-existing messages
exactly the same as messages that do exist.

The pre-existing "int keyword - non-existing message" parser test verifies
that no change in output has ocurred in this patch.

Change-Id: I0e32be14f1b420d7f222ac3c76e1cc266f912b69