Allow to pass null as cvvSelector, cardTypeSelector parameter

TypeError is thrown when null is passed as cvvSelector.
So, allow to pass null as cvvSelector and cardTypeSelector parameter.

```javascript
var creditly = Creditly.initialize(
    '.creditly-wrapper .expiration-month-and-year',
    '.creditly-wrapper .credit-card-number',
    null,
    '.creditly-wrapper .card-type'
);
```