Replace isset with null check for path_hash

Author: john-shaffer

src/PathInfo.php

@@ -118,7 +118,7 @@ public function getContentHash(): ?string {
     }
 
     public function getPathHash(): string {
-        if ( isset( $this->path_hash ) ) {
+        if ( $this->path_hash !== null ) {
             return $this->path_hash;
         }