If an opponent has a unit at a battlefield, does Shuriken Flip have to target, or can I just not choose to target and move my unit?

Yes — you can decline to target the enemy unit. Shuriken Flip says "Deal 2 to up to one enemy unit at a battlefield," and "up to" includes choosing zero even when a legal target is available (Rule 355.13). So you're never forced to hit their unit; you just move your own unit.

Two things to keep in mind: the "move a friendly unit" half is mandatory, so you need at least one friendly unit in play to cast it at all, and the enemy unit only qualifies if it's actually at a battlefield (one at their Base isn't a legal target anyway). Note also that Shuriken Flip has no Action/Reaction keyword, so it's Main Phase + Open State only.