Fix conditional rendering in ModalStack component

[Huderon]

Sorry for another single line PR. I kept seeing it in screenshots, though, and it was triggering me

[rauenzi]

Eh? What screenshots?

[Huderon]

Eh? What screenshots?

These ones. Mostly from Skye and Sarah, I believe.

[CalebBarnes]

Looks good. The issue was the modals.length && check will just return 0 if the check doesn't pass. Forcing it to be a boolean with !! is a valid fix.

{modals.length && <ModalLayer />}

You can also explicitly return null in cases like this:

{modals.length ? <ModalLayer /> : null}