Resolve #3408488 "Make unknownpathexcluder accept"

Closes #3408488

package_manager/src/PathExcluder/UnknownPathExcluder.php

@@ -112,11 +112,9 @@ final class UnknownPathExcluder implements EventSubscriberInterface {
       ->get('known_paths') ?: [];
     foreach ($safe_paths as $path) {
       if ($this->pathFactory->create($path)->isAbsolute()) {
-        $known_paths[] = $path;
-      }
-      else {
         throw new \InvalidArgumentException("'$path' must be relative to the project root.");
       }
+      $known_paths[] = $path;
     }
 
     // Search for all files (including hidden ones) in the project root. We need