module.exports = {
    'doctype-first': true,
    'doctype-html5': true,
    'attr-no-dup': true,
    'id-no-dup': true,
    'img-req-alt': true,
    'label-req-for': 'strict',
    'title-no-dup': true
};

@@ -129,28 +129,35 @@
 g-req-alt': 
-true
+'allownull'
 ,%0A    'label

Change img-req-alt value in validate preset to "allownull"