PHP 8.1 compatibility for https://github.com/facebookincubator/facebook-for-magento2

FACEBOOK_FOR_MAGENTO2_8_1_SUPPORT.patch

diff --git a/Helper/MagentoDataHelper.php b/Helper/MagentoDataHelper.php
index 992edc3..b2d94fa 100644
--- a/Helper/MagentoDataHelper.php
+++ b/Helper/MagentoDataHelper.php
@@ -478,7 +478,7 @@ class MagentoDataHelper extends AbstractHelper
     }
 
     private function hashValue($string){
-        return hash('sha256', strtolower($string));
+        return hash('sha256', strtolower($string ?? ''));
     }
 
     // TODO Remaining user/custom data methods that can be obtained using Magento.
diff --git a/Model/Feed/CategoryCollection.php b/Model/Feed/CategoryCollection.php
index de32055..0a39a2c 100644
--- a/Model/Feed/CategoryCollection.php
+++ b/Model/Feed/CategoryCollection.php
@@ -302,7 +302,7 @@ class CategoryCollection
             $this->fbeHelper->logException($e);
         }
         $this->fbeHelper->log("response from fb: ".$response);
-        $response_obj = json_decode($response, true);
+        $response_obj = json_decode($response, true) ?? [];
         if (array_key_exists('id', $response_obj)) {
             $set_id = $response_obj['id'];
             $this->saveFBProductSetID($category, $set_id);