Initial empty state is accepted

Hi! Thanks for this awesome package.

I tried copy-pasting the prompt example and I think there's an issue with the validation.

m := model{input: &prompt.Model{ValidateFunc: prompt.VFNotBlank}}

The validate function is VFNotBlank but you have to click any key first before the real-time validation functions correctly:

https://user-images.githubusercontent.com/13049130/180670558-ba9908dd-d93f-49fb-af0b-dbf55ccb024e.mov

Am I missing an option here?