Handle null arrays when retrieving array size

1. Check if an array is null before getting the array size
2. Add for loop testcases with null as the array to iterate
Fix testcase
Merge branch 'master' of https://github.com/DynamoDS/Dynamo into VMStability
Merge branch 'master' of https://github.com/DynamoDS/Dynamo into VMStability
Merge pull request #2516 from junmendoza/VMStability

Handle null arrays when retrieving array size
Merge branch 'master' of https://github.com/DynamoDS/Dynamo into Revit2015