Fix #4315: Disapproval messages aren't striped of spaces.

Set blank disapproval messages to null to prevent them from showing up
in wildcard searches.

test/factories/post_disapproval.rb

@@ -1,6 +1,7 @@
 FactoryBot.define do
   factory(:post_disapproval) do
     user
+    post
     reason { %w(breaks_rules poor_quality disinterest).sample }
     message { FFaker::Lorem.sentence }
   end