fix single_insert when a field is null (#477)

fix #449

Signed-off-by: Marc Poulhiès <dkm@kataplop.net>

include/dblayer/functions_mysql.inc.php

@@ -503,7 +503,7 @@ INSERT INTO '.$table_name.'
         $is_first = false;
       }
       
-      if ($value === '')
+      if ($value === '' || is_null($value))
       {
         $query .= 'NULL';
       }

include/dblayer/functions_mysqli.inc.php

@@ -594,7 +594,7 @@ INSERT INTO '.$table_name.'
         $is_first = false;
       }
       
-      if ($value === '')
+      if ($value === '' || is_null($value))
       {
         $query .= 'NULL';
       }