Fixes segfault
Fixes segfault

When audio_input is a nullptr, the current
program crashes due to trying to access
properties. This adds a null check.

Bug: 78236502
Change-Id: I14e1dbd0f2b34359670520f5405356f5bd4ab3d8

googlesamples/assistant-sdk-cpp,googlesamples/assistant-sdk-cpp,googlesamples/assistant-sdk-cpp