[11.x] Fix determining pivot timestamp column name(s) when parent relation missing one or both of timestamps (#53103)

Co-authored-by: Sergey Danilchenko <s.danilchenko@ttbooking.ru>

Author: daniser

src/Illuminate/Database/Eloquent/Relations/BelongsToMany.php

@@ -113,14 +113,14 @@ class BelongsToMany extends Relation
     /**
      * The custom pivot table column for the created_at timestamp.
      *
-     * @var string
+     * @var string|null
      */
     protected $pivotCreatedAt;
 
     /**
      * The custom pivot table column for the updated_at timestamp.
      *
-     * @var string
+     * @var string|null
      */
     protected $pivotUpdatedAt;
 
@@ -1478,7 +1478,7 @@ public function withTimestamps($createdAt = null, $updatedAt = null)
      */
     public function createdAt()
     {
-        return $this->pivotCreatedAt ?: $this->parent->getCreatedAtColumn();
+        return $this->pivotCreatedAt ?? $this->parent->getCreatedAtColumn() ?? Model::CREATED_AT;
     }
 
     /**
@@ -1488,7 +1488,7 @@ public function createdAt()
      */
     public function updatedAt()
     {
-        return $this->pivotUpdatedAt ?: $this->parent->getUpdatedAtColumn();
+        return $this->pivotUpdatedAt ?? $this->parent->getUpdatedAtColumn() ?? Model::UPDATED_AT;
     }
 
     /**