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
};

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

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