Xcode: Restore support for standalone IMPORTED_LOCATION_<CONFIG>

In commit 878ae03832 (macOS: IMPORTED framework: Honor SYSTEM target
property in all cases, 2023-08-27, v3.28.0-rc1~162^2) we broke support
for `IMPORTED_LOCATION_<CONFIG>` without `IMPORTED_CONFIGURATIONS`.
Previously it worked if the importing project's configurations match the
set of `IMPORTED_LOCATION_<CONFIG>` properties set.  Fix that case.

Fixes: #25506
Issue: #25515