CsipSetCoordinatorService: Fix race on bluetooth turning off

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

Bug: 324533075
Test: atest BluetoothCsipSetCooridinatorTest
Flags: Exempt, trivial fix.
Change-Id: Ib72decacfa397ee716d85cd2a4a0bfed02774a40