FIX: Make chat editor IDs not null (#18903)

Follow up to 766bcbc6840c9d665055441bcd77616b3a96e10e

Makes ChatMessage.last_editor_id and ChatMessageRevision.user_id
NOT NULL since they are always filled in now and the last commit
had a migration to backfill this data.

plugins/chat/spec/fabricators/chat_fabricator.rb

@@ -47,6 +47,7 @@ Fabricator(:chat_message_revision) do
   chat_message { Fabricate(:chat_message) }
   old_message { "something old" }
   new_message { "something new" }
+  user { |attrs| attrs[:chat_message].user }
 end
 
 Fabricator(:reviewable_chat_message) do