LE Audio: Replace invalid CsisClient::Get use by IsCsisClientRunning

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

Change-Id: I790bcd965612ce2f8b097daf8d6c853995f785a9
Test: TreeHugger