LE Audio: Replace invalid LeAudioClient::Get use by IsLeAudioClientRunning

LeAudioClient::Get asserts if the instance is null,
comparing it to nullptr is invalid as this case is imposible.
Use LeAudioClient::IsLeAudioClientRunning that do the proper check instead.

Test: TreeHugger
Fix: 269189182
Change-Id: I295230d0f634cbf8f78747eb7f1f7abb98ac13b4