fix for expected str instance, NoneType found

[ebdavison]

Closes: #149

Error encountered:

  File "/home/otcomm/otcomm/otcommenv/lib/python3.9/site-packages/mailer/engine.py", line 191, in send_all
    ", ".join(message.to_addresses))
TypeError: sequence item 1: expected str instance, NoneType found

Changes proposed in this PR:

• fix situation where email address is None when submitted from Django view to django-mailer

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