You're checking the layer for null, which it never will be.
What you want to do:
- get the first element of the intersect
- if there is no intersection, the intersect fs will be empty. calling First() on an empty fs will return null
- check that element for null
var evac_fs = FeatureSetByName($map,"RDNO Evacuation Areas")
var evac = First(Intersects(evac_fs, $feature))
return IIf(evac == null, "None", evac.Type)
Have a great day!
Johannes