Fix crash when invoking Form.ByLoftCrossSections with null input

### Issue

http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4511#

### Fix

Ultimately, the issue here is likely in the VM, when null is passed as an argument to a method expecting `object[]` or `object[][]`.  This fix needs to be made anyways, though, as without type info, replication will not work properly on `Loft.ByCrossSections`.   I will create a separate task for this after some more experimentation.  This also removes some dead private methods in the class.  

### Tests

The tests for `Form.ByLoftCrossSections` continue to work.

### Reviewers

@ikeough 

### Note

This is for Revit2015.  I will cherry-pick this if the reviewer approves.