Check for required value is now validates emptiness of next argument

Hi. There was an issue with checking required value:

1. Write in your php script `$GetOptionKit->add('t|test:', 'required value');`
2. Run it as follows:
`./script --test`
3. There is no exception on parsing args.
4. There, I fixed it!)

P.S. If I've misunderstood concept of 'option requires a value.', I'm sorry.