Fix null reference issues

There are currently 5 warnings being generated on build due to not handling possible null values correctly.

Affected lines in code:

Line 25 in 7c11cb1

return default;

Line 33 in 7c11cb1

return (T)systemTextJsonSerializer.Deserialize(stream, typeof(T), default);

Line 56 in 7c11cb1

    
           VirtualMachine virtualMachine = _azureApiService.GetAzVM(sessionHostItem.Properties?.ResourceId);

Line 65 in 7c11cb1

    
           avdHostData = _cosmosDbService.GetAvdHost(sessionHostItem.Properties?.ObjectId);