Null check before getting lacing attribute from xml node

This pull request is to fix defect [MAGN-4614  Fail to migrate and open attached file. Dynamo.Tests.CoreTests.CanOpenGoodFile is failing] (http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4614).

Function element in this dyn file doesn't have ``lacing`` attribute, so getting value from ``lacing`` attribute will throw a ``NullReference`` exception. Add null checking before getting this attribute.  

@ikeough , for you.