Prevent blank fields from being written when specifying a layout.

Hello, this is my first contribution to a project on GitHub, I hope I'm doing this correctly!

When a field is specified in the NLog.config, it is not written to the database unless it has a value. However, if a field has a custom layout, it is always written to the database, adding unnecessary clutter. For example, I'd like to add a field for the exception type, but I'm not always necessarily logging an exception.