Work around exception in EmailMessage().set_content('').
https://bugs.python.org/issue40597 has a fix committed for Python 3.8 and 3.9 which (inadvertently I hope) throws a ValueError
when attempting to set the content of a message part to the null string. I have submitted a PR to fix the fix to allow setting content to a null string as in the past, but I don't know if or when this will be accepted. In the mean time, django_mailman3/lib/scrub.py has part.set_content('')
in three places. These can be changed to part.set_content('\n')
with no change in the result to avoid the ValueError