PHP 8.1 compatibility

src/Storm/Model/Abstract.php

@@ -39,7 +39,7 @@ abstract class Storm_Model_Abstract {
   /**
    * @var string
    */
-  protected $_table_primary = null;
+  protected $_table_primary = 'id';
 
   /**
    * @var array
@@ -1491,4 +1491,4 @@ abstract class Storm_Model_Abstract {
   public function isFixedId() {
     return $this->_fixed_id;
   }
-}
\ No newline at end of file
+}