Avoid null dereference crash in test failure

We can't use ASSERT in these non-void functions, but we want to
stop executing them when our vector is empty.  Otherwise, we'll
crash with a null dereference and further confuse debugging for
users.

Bug: 362510140
Test: Local 'atest VtsHalVibratorTargetTest', while it still fails a couple tests, no longer segfaults.
Flag: EXEMPT for testing bug fix
Change-Id: I2deb674f0b13b0b511177fb95cc6fdc3160c49bc