Does Deflecting Swat work if you have no commander on the battlefield?

Deflecting Swat reads: 'If you control a commander, you may cast this spell without paying its mana cost.' This alternative cost condition checks only whether you control a commander at the time you cast the spell — it has nothing to do with whether your commander is attacking, blocking, or even whether it has ever been on the battlefield before.

Critically, 'control a commander' means any commander you own that is currently on the battlefield under your control (CR 903.3). If your commander is in the command zone, the graveyard, exile, or your hand, it is not on the battlefield and you do not control it for this purpose. So the alternative cost is NOT available if your commander is anywhere other than the battlefield.

However, if your commander IS on the battlefield, you can use the free casting option at any time Deflecting Swat could normally be cast — during combat, during your opponent's turn, whenever. You do not need to be in any particular phase or have your commander doing anything specific.

Concrete example: Your commander Atraxa is on the battlefield. An opponent targets your Atraxa with Swords to Plowshares. You cast Deflecting Swat without paying its {3}{R} cost (because you control a commander), redirecting the target to one of their own creatures. This works perfectly even though Atraxa is just sitting there doing nothing special.

If Atraxa had already been put into the command zone before that moment, you would NOT have access to the free cost and would need to pay {3}{R} normally to cast Deflecting Swat (CR 118.9a, 903.3).