I might be missing something but it seems it would evaluate to the same thing:
when customClass is falsy, it will evaluate to customClass
when customClass is truthy, it will evaluate to customClass
In all cases c gets passed the value of customClass.
This is the case in JavaScript and TypeScript, because short-circuits evaluate to the expression value and are not casted to booleans, like in some other languages.
Is there any reason you are not calling c with customClass directly, like c(customClass)?