Handle empty / undefined slice-type product variables uniformly.

When checking for the emptiness / non-existence of a product variable
having the type of a slice, check the length of this variable against
zero, instead of checking the variable itself against `nil`, as
empty / undefined list product variables may appear as nil or empty
lists -- the former approach works with both nil and empty lists,
while the latter only works with nil lists.

Test: m nothing
Bug: 159241638
Change-Id: Id436a13f8633bf698b64e6901662f67657034486