Classlib: Pstep: Fix unnecessary creation of an array on every iteration

    #val, dur = stream.next(inval) ? [nil, nil];

This means, **always** create the two-item array, even if `stream.next(inval)` is not nil. Non-nil is the normal case, by far more frequent. It should be conditional to create the array, and there's no reason why it shouldn't be a literal array.