Issue #3207893 by alexpott: Set system.css_js_query_string during install

core/modules/system/system.install

@@ -1313,6 +1313,9 @@ function system_install() {
     $site->set('name', 'Drupal');
   }
   $site->save(TRUE);
+
+  // Populate the dummy query string added to all CSS and JavaScript files.
+  _drupal_flush_css_js();
 }
 
 /**

core/tests/Drupal/FunctionalTests/Installer/InstallerTest.php

@@ -21,6 +21,7 @@ class InstallerTest extends InstallerTestBase {
    * Ensures that the user page is available after installation.
    */
   public function testInstaller() {
+    $this->assertNotNull(\Drupal::state()->get('system.css_js_query_string'), 'The dummy query string should be set during install');
     $this->assertSession()->addressEquals('user/1');
     $this->assertSession()->statusCodeEquals(200);
     // Confirm that we are logged-in after installation.