Fix test by not asserting stubbed result of LinkPermission.getName().

A test was asserting that LinkPermission.getName() returns null, which
was the stubbed behavior on Android prior to commit [1]. Since [1],
getName() returns "" instead (a different stubbed behavior), causing
the test to fail.

This CL changes the test to only assert that the method doesn't throw,
but doesn't assert any particular return value. This gets the test
passing for now; the test behavior may be reconsidered in the future
(bug 37380202).

Test: make cts && cts-tradefed run cts -m CtsLibcoreTestCases \
      -t libcore.java.nio.file.LinkPermissionTest
Test: Checked that nothing else broke:
      make cts && cts-tradefed run cts -m CtsLibcoreTestCases
Bug: 37380202

[1] https://r.android.com/#/c/platform/libcore/+/424841/
    (libcore commit bce13c50)
Change-Id: If2a58b3d59bb5ab75572f2ac4bf0598ad9353851