Verify check constraint existence before removing

This change fixes the `has_external_wiki` not null check constraint
rollback by checking if the constraint exists before removing it.

db/migrate/20210203092540_remove_has_external_wiki_constraint.rb

@@ -9,8 +9,10 @@ class RemoveHasExternalWikiConstraint < ActiveRecord::Migration[6.0]
 
   def up
     # This reverts the following migration: add_not_null_constraint :projects, :has_external_wiki, validate: false
+    if check_not_null_constraint_exists?(:projects, :has_external_wiki)
       remove_not_null_constraint :projects, :has_external_wiki
     end
+  end
 
   def down
     # no-op