Bluetooth: Mesh: Fix Config/Health Client async API

The previous PR was not fully baked, so this PR fixes issues by:
- Adding checks when dereferncing pointers stored in params.
- Clearing ack context if a return argument is NULL to not block and
wait for response.

Signed-off-by: Michał Narajowski <michal.narajowski@codecoup.pl>
4 files changed