HapClientService: Fix race on bluetooth turning off

When stressing BT ON/OFF it might happen that service is not available
when HapClientService is trying to get it.
In such case, let's be nice and do not throw exception

Bug: 324533075
Test: atest BluetoothHapClientTest
Flags: Exempt, trivial fix
Change-Id: I7519b7e5430acb1a3b1e64a8f862ee3f959bf6ea