Prefer null over empty string, for vendor contact info

rattail_corepos/importing/corepos.py

@@ -93,9 +93,9 @@ class VendorImporter(FromCOREPOS, importing.model.VendorImporter):
             'name': vendor.name,
             'abbreviation': vendor.abbreviation,
             'special_discount': special_discount,
-            'phone_number': vendor.phone,
-            'fax_number': vendor.fax,
-            'email_address': vendor.email,
+            'phone_number': vendor.phone or None,
+            'fax_number': vendor.fax or None,
+            'email_address': vendor.email or None,
         }