Can Stand United be cast without a target (specifically when you only control buffed units)?

Ruling: You can cast Stand United on a buffed unit. You cannot cast it if you don't have any units at all.

Nuances:

• If a spell requires a target, you must be able to select a valid target when the card is added to the chain (before paying cost and before resolution)
• If you cannot choose a target, the card becomes illegal and is removed from the chain
• If the illegal play was attempted from hand, it returns to your hand and the play is undone
• If Stand United was flipped from hidden and there's no longer a legal target, it goes to trash instead of returning to hand