fix(136521333): Handle None email_or_id in avatar code

Fixes https://www.pivotaltracker.com/story/show/136521333

avatars/avatars.py

@@ -77,7 +77,11 @@ class BaseAvatar(object):
         }
     """
     colors = self.colors
-    hash_value = hashlib.md5(email_or_id.strip().lower()).hexdigest()
+
+    # Note: email_or_id may be None if gotten from external auth when email is disabled,
+    # so use the username in that case.
+    username_email_or_id = email_or_id or name
+    hash_value = hashlib.md5(username_email_or_id.strip().lower()).hexdigest()
 
     byte_count = int(math.ceil(math.log(len(colors), 16)))
     byte_data = hash_value[0:byte_count]