Fix product variable zero value check

The zero value check was being done by using reflect.DeepEqual on a
field from the default product variables, but this results in
comparison against a random type when the product variables struct
for the module has been filtered down.  Luckily this will always
fail false, which just removed and optimization but left the
behavior correct.

Use reflect.IsZero instead, which is both faster and correct.

Test: variable_test.go
Change-Id: Ieaaa590c2788ca39230e6695397e8ba8d1c6c103