fix: don't crash if mail.outbox doesn't exist

[symwell]

Fixes #993

[samamorgan]

Change looks good, needs a test though!

[symwell]

It's unlikely I'll write a test.

[mskoenz]

This fix would be helpful, I currently use the workaround from @wouter-vdb

# see: https://github.com/pytest-dev/pytest-django/issues/993
@pytest.fixture(scope="function", autouse=True)
def _dj_autoclear_mailbox() -> None:
    pass