I've have made that modification suggested by @ventura but still doesn't work when 'hookActionProductQuantityUpdate' is triggered, this code does nothing:
if (isset($params['delta_quantity']) && (int) $params['delta_quantity'] === 0) { return; }
I have opened in the admin a product that was already out of stock, did nothing, and saved it (so no stock variation), but I received the email alert anyway
NOTE: PS v1.7.6