Fixed the headers of nunit-console for the 2.9.7 release

Pretty simple change to prevent confusion.
As you say, there is no difference and there isn't a common style guideline that I know of, but it is the style where I work so it has become habit and I think it makes you more likely to use other Empty calls like `Guid.Empty` or `EventArgs.Empty`. Pretty minor if you prefer "" I am fine with either.