Can you "spend" a focus token to trigger an effect like the R4 Agromech even if there's nothing to spend it on? I believe there's a pilot that can do that to trigger his ability but did that set a precedent for other action to trigger?

Yes. You can spend focus tokens to modify zero results. You need to change every eyeball to a hit when you spend it. IT doesn't matter if nothing ends up getting modified.

You do however need something to actually allows you to spend focus tokens though. For example, Garven Dreis (with no upgrades) that isn't attacking or defending in a round is unable to spend his focus token to be able to assign one to another friendly ship.

You could also spend a Target Lock and roll nothing. (Contrary to Focus where you have to change de result)

And you can spend any number of them even if this is useless...

It's not a precendent, more like, there is nothing in the rules that say that you can't spend something if it would not be usefull.

But like WWHSD said, there is window of opportunity where you can do it. You can't just say, here I'd to spend my token at start of combat phase.

When there is a limit, it is normaly written on the card, example Keyan Farlander, it is said when attacking you can spend 1 stress token. So you can't spend more then one per attack.

When there is a limit, it is normaly written on the card, example Keyan Farlander, it is said when attacking you can spend 1 stress token. So you can't spend more then one per attack.