ml: fix vts called on a disengaged value issue

The variable extraParams in aidl vts is a optional, when it is empty, the extraParams->set will fail on error: optional operator-> called on a disengaged value.

BUG=b:253138038
TEST=USE="-vendor-nnhal" FEATURES=nostrip emerge-asurada
aosp-frameworks-ml-nn-vts && cros_nnapi_vts_aidl

Change-Id: I34bb2d02fc78c5d031a992d2a5ea9c8d92f370ef
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/hardware/interfaces/neuralnetworks/+/3947888
Tested-by: Jack Shen <jackshen@google.com>
Commit-Queue: Jack Shen <jackshen@google.com>
Reviewed-by: Jim Pollock <jmpollock@chromium.org>