Add checks for nullptr in Camera2Client::disconnect

If Camera2Client::initialize fails in CameraService::connectHelper,
we call disconnect immediately afterwards. Some objects may have been
constructed, while others might not have been. Check for null before
dereferencing their pointers.

Bug: 347116037
Flag: EXEMPT bugfix
Test: CtsCameraTestCases
Change-Id: I5ad2551de1928d6590216f8fafcffa432b66ff5c