Check for photoset's has_requested_photos before using

Only photos owned by the user will be returned in has_requested_photos,
otherwise the data is left off. This was resulting in the following error:

> Fatal error: Uncaught TypeError: in_array(): Argument #2 ($haystack) must be of type array, null given

Author: samwilson

src/PhotosApi.php

@@ -474,7 +474,8 @@ public function getSets($photoIds, $userId = null)
             }
             foreach ($sets['photoset'] as $photoset) {
                 foreach ($photoIds as $photoId) {
-                    if (in_array($photoId, $photoset['has_requested_photos'])) {
+                    // has_requested_photos is only set if the user owns the photo.
+                    if (in_array($photoId, $photoset['has_requested_photos'] ?? [])) {
                         $out[] = $photoset;
                     }
                 }