riposte with no unit

No — Riposte needs both a friendly unit and a spell as targets, so with zero units in play you can't play it at all.

Both choices in "Choose a friendly unit and a spell" are mandatory targets, and all targets must be legal when the spell is finalized on the chain (Rule 358.1/355.9.b; FAQ #11923). You can't play it targeting only the spell and skip the buff.

Note the flip side: if you do have a unit when you play it and that unit becomes illegal before Riposte resolves, the counter still happens — only the +Might instruction is ignored (Rule 359.3.e.8).