Fixed error thrown when element missing name

I have the following code

```html
<form id="myform">
    <input type="text" name="name" placeholder="Full Name" required />
    <label class="checkbox">
        <input type="checkbox" required />
        I agree not to send any unsolicited email, or send email to addresses purchased, rented or otherwise obtained from a third party. I understand that port 25 is blocked by default and I have to request that it be opened. I also understand that if I violate the rules set by Server Blaze that port 25 will be closed.
    </label>
    <button type="submit">Done</button>
</form>
```

and in JavaScript I have this

```javascript
$("#myform").validate()
```

The code always errors out and submits regardless of if a name is entered into the name field or not.  I have tracked the error down to this line:

https://github.com/jzaefferer/jquery-validation/blob/master/jquery.validate.js#L729

With this PR, I fix the error in the code by skipping the name check if it doesn't exit.