Accept empty message in retry_if_exception_message (#651)

retry_if_exception_message validated its message/match arguments with
truthiness checks (if message and match, if not message and not match).
An empty string is a valid message target — it matches exceptions whose
str() is empty, e.g. a bare RuntimeError() — but 'not message' is True
for '', so retry_if_exception_message(message='') wrongly raised
TypeError('missing 1 required argument'). _check had the same problem:
'if self.message:' skipped the empty-message branch.

Replace the truthiness guards with 'is None' checks so an empty message
is accepted and matched. This makes every message matchable, not every
message except the empty one.