This came up at a tournament yesterday. we both agreed on the ruling but figured it'd would be worth asking on here about.

Onar uses an action to Rush.
This causes him to enter an adjacent space next to a Jawa.
The Jawa plays Dirty Trick.

We decided that Onar could just opt to use the second option of the card; become stunned, which would have no effect because he is Immune.

The alternative ruling would be that he must pick the first option because he can't meet the requirements of the second option.

What's the right call here?

Dirty Trick does not have any explicit prerequisites (costs would be implicit prerequisites) in choosing options (like for example Fenn's Suppressive Fire). The alternatives are not costs, they are consequencies. For example if the figure is stunned already, it can still choose to become stunned. So, I see that Onar could choose to become Stunned, and then ignore it due to Immune. It would be different if "becoming Stunned" were a cost to perform an ability.

Onar could also use To the Limit without becoming stunned. Again, becoming stunned is a consequence and not a cost.

Unless explicitly stated, you can choose any option even if you cannot fully resolve it. (Note that it is explicitly stated in the rules that in the campaign heroes cannot choose to suffer strain over their endurance.)

There's no official ruling as far as I know. To be sure, it is simply best not to play Dirty Trick on Onar.