Add null checking when deleting hotword detectors

Nullpointer exception could be thrown when the system does not specify
VisualQueryDetector and is trying to shutdown all hotword detectors.
This can lead to the destroy of hotword detectors to fail which further
leads to app crash when a second detector is created and further cause
memory leak.

Bug: 283264602
Test: atest CtsVoiceInteractionTestCases
Change-Id: If1a6b11ccfae6e3002e5587d0eb7de80943583c3