Issue #3508027 by grimreaper: Do not override existing config on install

Closes #3508027

robotstxt.install

@@ -11,7 +11,10 @@ use Drupal\Core\Routing\RequestHelper;
  * Implements hook_install().
  */
 function robotstxt_install() {
-  $content = '';
+  $config = \Drupal::configFactory()->getEditable('robotstxt.settings');
+  if ($config->get('content')) {
+    return;
+  }
 
   // List of candidates for import.
   $files = [
@@ -20,6 +23,7 @@ function robotstxt_install() {
     \Drupal::service('extension.list.module')->getPath('robotstxt') . '/robots.txt',
   ];
 
+  $content = '';
   foreach ($files as $file) {
     if (file_exists($file) && is_readable($file)) {
       $content = file_get_contents($file);
@@ -27,7 +31,7 @@ function robotstxt_install() {
     }
   }
 
-  \Drupal::configFactory()->getEditable('robotstxt.settings')->set('content', $content)->save();
+  $config->set('content', $content)->save();
 }
 
 /**