TypeError in handling moderator requests.

[Mark Sapiro]

The fix for https://bugs.launchpad.net/mailman/+bug/1949403 can cause this issue in the admindb interface if a list has no moderator password

Traceback (most recent call last):
  File "/mailman/mailman-${domainslug}/scripts/driver", line 117, in run_main
    main()
  File "/mailman/mailman-${domainslug}/Mailman/Cgi/admindb.py", line 342, in main
    print doc.Format()
  File "/mailman/mailman-${domainslug}/Mailman/htmlformat.py", line 352, in Format
    output.append(Container.Format(self, indent))
  File "/mailman/mailman-${domainslug}/Mailman/htmlformat.py", line 267, in Format
    output.append(HTMLFormatObject(item, indent))
  File "/mailman/mailman-${domainslug}/Mailman/htmlformat.py", line 53, in HTMLFormatObject
    return item.Format(indent)
  File "/mailman/mailman-${domainslug}/Mailman/htmlformat.py", line 445, in Format
    % csrf_token(self.mlist, self.contexts, self.user)
  File "/mailman/mailman-${domainslug}/Mailman/CSRFcheck.py", line 53, in csrf_token
    mac = sha_new(secret + `issued`).hexdigest()
TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'