fix: getQualified{Created,Updated}AtColumn never returning null (#54568)

Author: calebdw

src/Illuminate/Database/Eloquent/Concerns/HasTimestamps.php

@@ -161,7 +161,9 @@ public function getUpdatedAtColumn()
      */
     public function getQualifiedCreatedAtColumn()
     {
-        return $this->qualifyColumn($this->getCreatedAtColumn());
+        $column = $this->getCreatedAtColumn();
+
+        return $column ? $this->qualifyColumn($column) : null;
     }
 
     /**
@@ -171,7 +173,9 @@ public function getQualifiedCreatedAtColumn()
      */
     public function getQualifiedUpdatedAtColumn()
     {
-        return $this->qualifyColumn($this->getUpdatedAtColumn());
+        $column = $this->getUpdatedAtColumn();
+
+        return $column ? $this->qualifyColumn($column) : null;
     }
 
     /**