Fix approved column being set to nil instead of false (#10642)

Fix https://github.com/tootsuite/mastodon/pull/10621#issuecomment-487316619

app/models/user.rb

@@ -282,7 +282,7 @@ class User < ApplicationRecord
   end
 
   def external?
-    @external
+    !!@external
   end
 
   def sanitize_languages