Fix annoying warnings in debugger

This pull request fixes the annoying message in "Visual Studio" if Dynamo is launched with debugger attached. The warning looks something like these:

```
System.Windows.Data Error: 6 : 'TargetDefaultValueConverter' converter failed to convert value '' (type 'String'); fallback value will be used, if available. BindingExpression:Path=IconResourcePath; DataItem='StartPageListItem' (HashCode=35975003); target element is 'Image' (Name=''); target property is 'Source' (type 'ImageSource') NotSupportedException:'System.NotSupportedException: ImageSourceConverter cannot convert from System.String.
```

and
```
System.Windows.Data Error: 23 : Cannot convert '' from type 'String' to type 'System.Windows.Media.ImageSource' for 'en-US' culture with default conversions; consider using Converter property of Binding. NotSupportedException:'System.NotSupportedException: ImageSourceConverter cannot convert from System.String.
```