Simplify missing whole_static_libs checking

Whole_static_libs required custom error checking when
AllowMissingDependencies was set because it could end up depending
on an empty list of objects, which would leave nothing in the
dependency tree that had been replaced with an ErrorRule.
Reuse the prebuilts case to depend on the .a file when there
are no objects and remove the custom error handling.

Test: TestEmptyWholeStaticLibsAllowMissingDependencies
Change-Id: Ic3216235f7e5ae8b5b6ab31ef2ca35c3994d82aa