Have you looked at the NULL Value Option and NULL Replacement Value when you edit a field in the BO Mapper? Both of these typically allow you to account for NULLs however you prefer. You can set it to Use Nullable Generics (where it can), or to replace a NULL with a specific value (i.e. "" or String.Empty). There are several options there, so you may want to take a look and see if any of them suit your fancy 
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