CatalogItem executed with no args but a REQUIRED args_hint

@ueffel [noticed](https://gitter.im/Keypirinha/Keypirinha?at=5bbe3052271506518deb8107) that KP wrongly allows an item to be executed from the "args" search step (i.e. after having hit the `Tab` key), even though it has got a `REQUIRED` `args_hint`.