[libomptarget] Make omp_target_is_present checks storage instead of zero length array.

Consider checking whether a pointer has been mapped can be achieved via omp_get_mapped_ptr.
omp_target_is_present is more needed to check whether the storage being pointed is mapped.
This restore the old behavior of omp_target_is_present before D123093
Fixes https://github.com/llvm/llvm-project/issues/54899

Reviewed By: jdenny

Differential Revision: https://reviews.llvm.org/D123891