AutoMapping and Default Values

I am auto mapping my objects, but wish for my strings to default to empty strings, not null.
I tried making a IAutoMappingOverride with this:

        mapping.Map(x => x.NameMiddle).Default(string.Empty);

When the NameMiddle value is never set before SaveOrUpdate() call I still get generated SQL that attempts insert a null value.

How do I set default values for all strings, or for specific properties?
Even just pointing me to a resource that worked for you would help, I've found next to nothing (possibly I'm just not searching for the proper words).