add migration nullable oauth_app in users table

db/migrate/20181130103140_allow_nulls_in_user_id.rb

@@ -0,0 +1,16 @@
+require 'carto/db/migration_helper'
+
+include Carto::Db::MigrationHelper
+
+migration(
+  Proc.new do
+    alter_table :oauth_apps do
+      set_column_allow_null :user_id
+    end
+  end,
+  Proc.new do
+    alter_table :oauth_apps do
+      set_column_not_null :user_id
+    end
+  end
+)