broadcaster: Add missing null checks at client side

These can be already checked at the frameworks side.

Bug: 150670922
Tag: #feature
Test: atest CtsBluetoothTestCases:android.bluetooth.cts.BluetoothHapClientTest
Sponsor: jpawlowski@
Change-Id: I1b3d68594f5c96343080d4cf693bd41cce8be581