Accept empty message in retry_if_exception_message

[gaoflow]

Problem

retry_if_exception_message validates its two mutually-exclusive arguments
with truthiness checks:

if message and match:            # both given
    raise TypeError(...)
if not message and not match:    # neither given
    raise TypeError(...)
...
self.match = re.compile(match) if match else None
...
def _check(self, exception):
    if self.message:             # skips message == ""
        return self.message == str(exception)

An empty string is a valid message target — it matches exceptions whose
str() is empty, e.g. a bare raise RuntimeError() (str(RuntimeError()) == "").
But not message is True for "", so:

retry_if_exception_message(message="")
# TypeError: retry_if_exception_message() missing 1 required argument 'message' or 'match'

The result is an asymmetry: you can match every exception message except
the empty one. _check's if self.message: would drop the empty-message branch
as well, falling through to assert self.match is not None.

Fix

Replace the truthiness guards with is None checks, so an empty message is
accepted and matched like any other. retry_if_not_exception_message inherits
the same constructor and is fixed by the same change.

Tests

Added test_retry_if_exception_message_empty_message and
test_retry_if_not_exception_message_empty_message: construct with message=""
and drive the predicate against a bare RuntimeError() (matches) and a
RuntimeError("boom") (does not), plus the inverse for the not-variant. Both
fail before the change (TypeError at construction) and pass after. The
existing test_..._negative_no_inputs (passing no arguments must raise) still
passes.

Full suite: 167 passed, 1 skipped, 12 subtests. ruff check / ruff format --check clean.