Check if needle is empty before using strpos.

Author: reense

lib/internal/Magento/Framework/Filesystem/Directory/DenyListPathValidator.php

@@ -71,7 +71,7 @@ public function validate(
 
         foreach ($this->fileDenyList as $file) {
             $baseName = pathinfo($actualPath, PATHINFO_BASENAME);
-            if (stripos($baseName, $file) !== false || preg_match('#' . "\." . $file . '#', $fullPath)) {
+            if ('' === $needle || false !== strpos($haystack, $needle) || preg_match('#' . "\." . $file . '#', $fullPath)) {
                 throw new ValidatorException(
                     new Phrase('"%1" is not a valid file path', [$path])
                 );