Various fixes

- Added a test case for List operation containing a null element (new issue MAGN-2296)
- Changed the migration function of Compare nodes into DSCoreNodes.Compare.*
- Changed the input type of Compare nodes into "var"
- Added input type casting for "List.Sublists" to be able to support non-integer list indices (fix for MAGN-2283)
- Marked the test case "Dynamo.Tests.CoreDynTests.Formula" as inconclusive (resolved MAGN-1792)