Add additional validation for the admin register endpoint. (#8837)

Raise a proper 400 error if the `mac` field is missing.

changelog.d/8837.bugfix

+Fix a long standing bug in the register admin endpoint (`/_synapse/admin/v1/register`) when the `mac` field was not provided. The endpoint now properly returns a 400 error. Contributed by @edwargix.

synapse/rest/admin/users.py

@@ -420,6 +420,9 @@ class UserRegisterServlet(RestServlet):
         if user_type is not None and user_type not in UserTypes.ALL_USER_TYPES:
             raise SynapseError(400, "Invalid user type")
 
+        if "mac" not in body:
+            raise SynapseError(400, "mac must be specified", errcode=Codes.BAD_JSON)
+
         got_mac = body["mac"]
 
         want_mac_builder = hmac.new(