Make sure email-sender is defined before sending email

If sender is not defined, use email-username:

[2022-01-14 13:42:34,465][ERROR  ][127.0.0.1][anonymous][ThreadPoolExecutor-0_0][apscheduler.executors.default] Job "EmailClient.send_mail (trigger: date[2022-01-14 13:42:32 UTC], next run at: 2022-01-14 13:42:32 UTC)" raised an exception
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apscheduler/executors/base.py", line 125, in run_job
    retval = job.func(*job.args, **job.kwargs)
  File "/usr/lib/python3/dist-packages/rdiffweb/core/notification.py", line 182, in send_mail
    conn.sendmail(self._email_from, to_user.email, msg.as_string())
  File "/usr/lib/python3.9/smtplib.py", line 876, in sendmail
    (code, resp) = self.mail(from_addr, esmtp_opts)
  File "/usr/lib/python3.9/smtplib.py", line 540, in mail
    self.putcmd("mail", "FROM:%s%s" % (quoteaddr(sender), optionlist))
  File "/usr/lib/python3.9/smtplib.py", line 154, in quoteaddr
    if addrstring.strip().startswith('<'):
AttributeError: 'NoneType' object has no attribute 'strip'