Resolve #3408488 "Make unknownpathexcluder accept"

Closes #3408488

package_manager/src/PathExcluder/UnknownPathExcluder.php

@@ -76,8 +76,6 @@ final class UnknownPathExcluder implements EventSubscriberInterface {
       return;
     }
 
-    $project_root = $this->pathLocator->getProjectRoot();
-
     // To determine the files to include, the installed packages must be known,
     // and that requires Composer commands to be able to run. This intentionally
     // does not catch exceptions: failed Composer validation in the project root
@@ -85,11 +83,12 @@ final class UnknownPathExcluder implements EventSubscriberInterface {
     // Note: the call to ComposerInspector::getInstalledPackagesList() would
     // also have triggered this, but explicitness is preferred here.
     // @see \Drupal\package_manager\StatusCheckTrait::runStatusCheck()
+    $project_root = $this->pathLocator->getProjectRoot();
     $this->composerInspector->validate($project_root);
 
     // The vendor directory and web root are always included in staging
-    // operations, along with `composer.json`, `composer.lock`, and any files
-    // scaffold files provided by `drupal/core`.
+    // operations, along with `composer.json`, `composer.lock`, and any scaffold
+    // files provided by Drupal core.
     $include = [
       $this->composerInspector->getConfig('vendor-dir', $project_root),
       $web_root,
@@ -129,10 +128,8 @@ final class UnknownPathExcluder implements EventSubscriberInterface {
     $project_root = $this->pathLocator->getProjectRoot();
     $packages = $this->composerInspector->getInstalledPackagesList($project_root);
     $extra = Json::decode($this->composerInspector->getConfig('extra', $packages['drupal/core']->path . '/composer.json'));
-    if (isset($extra['drupal-scaffold']['file-mapping'])) {
-      return array_keys($extra['drupal-scaffold']['file-mapping']);
-    }
-    return [];
+
+    return array_keys($extra['drupal-scaffold']['file-mapping'] ?? []);
   }
 
 }