Added handling of undefined classes

Undefined or other falsy arguments will now be ignored instead of causing the function to throw an error.

In React, it is a common pattern to extract ```className``` from ```this.props``` like os:

```
var {className, ...other} = this.props;
```

With this pull request, the ```className``` variable can be merged with ```class-merge``` without having to check if it is actually defined in ```this.props```