Specialize `DefaultEmptyFn` for `std::string`.

The default implementation for `IsEmpty()` turns out
to be very slow.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 181943478
Change-Id: I0cbeb4ff942a9a0ad0c6130eec7cc096cf3d564e
1 file changed