Restore DATABASE_URL even if it's nil in connection_handler test

- We have to restore DATABASE_URL to its previous state irrespective of
  previous value is nil or not

activerecord/test/cases/connection_adapters/connection_handler_test.rb

@@ -14,7 +14,7 @@ def setup
       end
 
       def teardown
-        ENV["DATABASE_URL"] = @previous_database_url if @previous_database_url
+        ENV["DATABASE_URL"] = @previous_database_url
       end
 
       def test_string_connection