DEV: Allow DROP NOT NULL in pre-deploy migrations (#20775)

Our SafeMigrate system is designed to prevent tables/columns being dropped in pre-deploy migrations. Its regex-based detection was triggering incorrectly on `ALTER COLUMN DROP NOT NULL`.

spec/fixtures/db/migrate/drop_not_null/20990309014015_drop_not_null.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class DropNotNull < ActiveRecord::Migration[5.1]
+  def up
+    change_column_null :users, :username, true
+  end
+
+  def down
+    raise "not tested"
+  end
+end