Wrong handling of deleting strings with comment

Delete the German string for "Yes" with the ResX Manager.

Works well for entries without a comment. The ResX Manager deletes the `` node:

```xml

Ja

```

But when the German stings has a comment the string is set to an empty string:

```xml

Ja
My sample comment.

```

deleting of "Ja" results in:

```xml

My sample comment.

```

In this case the fallback mechanism of the resource manager does not work, because the string is an empty string but it should be `null`.