Check that ScanClient#stats is not null before dereferencing it.

This check already is in place for most usages in ScanManager. It's possible for stats to be null even in the case of registering and starting a scanner.

Bug: 338904991
Flag: EXEMPT, null pointer checks
Test: atest BluetoothInstrumentationTests, atest BumbleBluetoothTests
Change-Id: I2a8b0394817b71760789ab38cb3d92dabba7bfb2