Add nulls not distinct to unique_user_data constraint

Add nulls not distinct to unique_user_data constraint to prevent double inserts for system accounts

Author: amvanbaren

server/src/main/resources/db/migration/V1_49__Unique_User.sql

@@ -0,0 +1,2 @@
+ALTER TABLE user_data DROP CONSTRAINT unique_user_data;
+ALTER TABLE user_data ADD CONSTRAINT unique_user_data UNIQUE NULLS NOT DISTINCT (provider, login_name);