Identity: Add VTS test to check empty and semi-empty requests work properly.

Some IC applications may perform two requests - one to get data
elements and a second empty request. The latter is to e.g. get an
empty DeviceSignedItems and corresponding MAC.

Extend VTS tests to check that the HAL does this correctly both for
the completely empty request and also for a request with an empty
namespace.

Bug: 160966911
Test: atest VtsHalIdentityTargetTest
Merged-In: I3205f2c0ded2ea315857438a3114ddcf8ef557f9
Change-Id: Ib718e6f0f0b287e39ff7dd3db5335cec1bc1804e