Use an empty iterable instead of None in the valid upload mime-types settings when there's no required mime type, in order to simplify other code.

- Legacy-Id: 14778

ietf/settings.py

@@ -742,7 +742,7 @@ MEETING_VALID_UPLOAD_EXTENSIONS = {
 MEETING_VALID_UPLOAD_MIME_TYPES = {
     'agenda':       ['text/plain', 'text/html', ],
     'minutes':      ['text/plain', 'text/html', 'application/pdf', ],
-    'slides':       None,
+    'slides':       [],
     'bluesheets':   ['application/pdf', 'text/plain', ],
 }