should not to_s the possibly nil value

activerecord/test/cases/ar_schema_test.rb

@@ -55,11 +55,11 @@ def test_migration_adds_row_to_migrations_table
 
       assert_equal "123001", rows[0]["version"]
       assert_equal "", rows[0]["name"]
-      assert_match(/^2\d\d\d-/, rows[0]["migrated_at"].to_s)
+      assert_not_nil(rows[0]["migrated_at"])
 
       assert_equal "123002", rows[1]["version"]
       assert_equal "add_magic_power_to_unicorns", rows[1]["name"]
-      assert_match(/^2\d\d\d-/, rows[1]["migrated_at"].to_s)
+      assert_not_nil(rows[1]["migrated_at"])
     end
 
     def test_define_clears_schema_migrations