Fix broken test case TestRecordAndDeleteModelsWithEmptyInput

Notice test case ``Dynamo.Tests.CoreTests.TestRecordAndDeleteModelsWithEmptyInput`` is broken. In this test case, the null reference exception is thrown out from this line ``NodeModel node = ViewModel.Model.Nodes[0];`` which I believe is a copy/paste error (it was added in commit https://github.com/DynamoDS/Dynamo/commit/64a774fc310c0454055dc7c338744e0bc1612ea4). 

@sharadkjaiswal  please review the change. Thanks.