Fix missing query string when system.css_js_query_string is NULL

When we first time enable the option: Locally cache tracking code file, we got the local js query string none:
/sites/default/files/google_analytics/analytics.js?

The D8 code should be change from

$query_string = '?' . \Drupal::state()->get('system.css_js_query_string') ?: '0';

to

$query_string = '?' . (\Drupal::state()->get('system.css_js_query_string') ?: '0');

Then it will be the same as the D7 version:

$query_string = '?' . variable_get('css_js_query_string', '0');