Use reflect.Zero(type) to get value to clear field

Previously, the common value extraction code used an empty structure
to get the value to use to clear a field whose value is common. This
change removed the structure and used reflect.Zero(..) to get the
value instead.

Bug: 142935992
Test: m nothing
Change-Id: Ibd5103dacb86e7754a786356c0d15ffbde7f98bf