Don't bother running the query without an address

This should solve the following in admin logs:
"WARNING in nginx: Invalid user 'xxxx': (builtins.ValueError)
invalid email address (no "@")"

core/admin/mailu/models.py

@@ -636,7 +636,7 @@ in clear-text regardless of the presence of the cache.
     @classmethod
     def get(cls, email):
         """ find user object for email address """
-        return cls.query.get(email)
+        '@' in email and return cls.query.get(email)
 
     @classmethod
     def login(cls, email, password):