Fix case when relative path is empty

Author: coderimus

app/code/Magento/MediaGallery/Plugin/Wysiwyg/Images/Storage.php

@@ -122,6 +122,6 @@ private function getMediaDirectoryRelativePath(string $path): string
     {
         $relativePath = $this->filesystem->getDirectoryRead(DirectoryList::MEDIA)->getRelativePath($path);
 
-        return (false === strpos($relativePath, '/')) ? '/' . $relativePath : $relativePath;
+        return ($relativePath && false === strpos($relativePath, '/')) ? '/' . $relativePath : $relativePath;
     }
 }