Fixed null type error when converting enum to string

Author: Josh Ralph

src/CacheKey.php

@@ -446,14 +446,17 @@ protected function recursiveImplode(array $items, string $glue = ",") : string
         return $result;
     }
 
-    private function processEnum(\BackedEnum|\UnitEnum|Expression|string $value): string
+    private function processEnum(\BackedEnum|\UnitEnum|Expression|string|null $value): string
     {
         if ($value instanceof \BackedEnum) {
             return $value->value;
         } elseif ($value instanceof \UnitEnum) {
             return $value->name;
         } elseif ($value instanceof Expression) {
             return $this->expressionToString($value);
+        } elseif (is_null($value)) {
+            // @todo - this is just a patch on our side, for what is resolved by https://github.com/laravel/framework/pull/48606 being merged.
+            return 'null';
         }
 
         return $value;