Fix issue with deduplication of connectors.

unit is set to emptystring which doesen't deduplicate with the none 
present when unit is not set

src/wireviz/Harness.py

@@ -377,7 +377,7 @@ def bom(self):
                                + (f', {connector.pincount} pins' if connector.show_pincount else '')
                                + (f', {connector.color}' if connector.color else ''))
                 bom_entries.append({
-                    'item': description, 'qty': 1, 'unit': '', 'designators': connector.name if connector.show_name else None,
+                    'item': description, 'qty': 1, 'unit': None, 'designators': connector.name if connector.show_name else None,
                     'manufacturer': connector.manufacturer, 'mpn': connector.mpn, 'pn': connector.pn
                 })