fix MAGN-3653 Wrong migration for String.Join, crashes on copy paste.

[MAGN-3653](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-3653)
The migration error is caused by the logical change in String.Join node. "Join" has been changed from a DsFunction node to a VarArgFunctionNode. I have change the migration path to correct one.