VB -> C#: WithEvents fields without initializers cause a NullReferenceException

A WithEvents field in a VB class that doesn't have an initializer causes a NullReferenceException in VisitFieldDeclaration in VisualBasicConverter.NodeVisitor.  Input code example:

```vbnet
Class Class1
    WithEvents MyEventClassInstance As MyEventClass
End Class
```
Pull request to follow.