Check for null before building string node.

This pull request adds a null check in the ReferenceSelection AST building, which passes out a null node if the an id cannot be acquired from revit for a given reference.

A larger task has been opened to harden all ASTFactory methods:
http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5096

Requires merge:
- [ ] Revit2015

PTAL:
@Steell 
Do we want to silently return null, or do we need to push a warning to the user?