can i play riposte if theres no target spell to counter?

No — Riposte can't be played without a spell on the chain. Its text says "Choose a friendly unit and a spell," and both choices are mandatory targets that must be legal before the spell can be finalized on the chain (FAQ 11923). You also need a friendly unit, so you can't play it with a spell available but no valid unit target.

Note the spell doesn't have to be an enemy spell — countering your own spell is legal (FAQ 453), so you can still play Riposte in response to your own spell if you want the +[M] buff.

If you're eyeing it just to trigger "when you play a spell" effects, that doesn't work either — a spell with an unmakeable mandatory target can't be played at all (Rule 358.1).