Don't attempt validation on an empty rule

I had a case where I had rules set as:
```
array(
  'title' => 'required',
  'blurb' => ''
)
```

When I manually do a validation check with the following:

```
$validator = \Validator::make($this->attributes, $rules);
$boards[$boardName] = $validator->passes();
```

If I have the value "myblurb" stored for the items blurb value, I get an error telling me the method `validateMyblurb` does not exist, it seems that this function calls itself in cases where no validation rules are set and tries to create a $method based on the concatenation of 'method' and the original value

This check simply stops this from happening.