GenericTable: better handling of null values for timestamps

models/GenericTable.php

@@ -226,8 +226,8 @@ class GenericTable
 			else
 				$pattern = $options['pattern'];
 
-			assert(isset($rowData[$options['value']]));
-			if (!is_numeric($rowData[$options['value']]))
+			assert(array_key_exists($options['value'], $rowData));
+			if (isset($rowData[$options['value']]) && !is_numeric($rowData[$options['value']]))
 				$timestamp = strtotime($rowData[$options['value']]);
 			else
 				$timestamp = (int) $rowData[$options['value']];