Fix get_user_first_group for user with no groups

If the email is defined in user-groups.yaml but has no groups, groups is
assigned None and len(groups) throws an exception.

Intuitively, one would expect groups to default to [] rather than None
because [] is passed as the second argument to Dictionary.get, but this
default only applies if the key is not found in the dictionary. In this
case the key is defined but has a value of None.

src/worker.py

@@ -104,7 +104,7 @@ def read_user_groups():
 def get_user_first_group(email):
     user_groups_yaml = read_user_groups()
     groups = user_groups_yaml.get("users", {}).get(email, [])
-    if len(groups): return groups[0]
+    if groups != None and len(groups): return groups[0]
     return "default"