gum input: implement check for empty input

[D3vil0p3r]

Is your feature request related to a problem? Please describe.
Currently I don't see a way in gum to prompt again the user to insert a text if the input is empty.

For example I have:

gum input --placeholder "Please enter your username"

I would like that, if the user inserts an empty value, gum should ask again for the input.

I think it could be easy to implement, for example adding a further argument/option for this purpose.