nobu_the_bard says... #2
No. It must have a target to be cast.
The target must be a spell on the stack. You cannot cast Lightning Bolt if there are no valid targets either (maybe every player and creature has shroud somehow).
If you are desperate, though, you can counter your own spells. I've been there now and again.
Tyrant-Thanatos says... Accepted answer #1
No, a card that designates a target or a number of targets must have legal targets for you to declare in order to cast them. Hex for instance, literally cannot be cast if there are less than six creatures on the field that it can target. Even if there are say, 8 creatures on the field, but 4 of them have Shroud and/or Protection from Black, you cannot cast Hex.
If a card with multiple targets has some of its targets become illegal before it resolves, it will resolve as much as it can, however if a card has all of its targets become illegal, it will be countered.
September 15, 2017 3:23 p.m. Edited.