Allow cc.LinkabeInterfaces to request "" variants.

cc.LinkableInterfaces should be able to request if an empty variant is
required from the LinkageMutator. We were previously assuming that if only
static or shared libraries were being built, then we didn't need the
empty variant. However this should be explicit to handle cases where the
default set of variants being created may not include both static and
shared libraries.

Bug: 144861059
Test: m -j crosvm.experimental
Change-Id: I66724bdc16ff350b06dfa4d049c82f33019979a5