test non-numeric serials are handled correctly for ranges

InvenTree/stock/tests.py

@@ -396,6 +396,9 @@ class VariantTest(StockTest):
         item.save()
         self.assertEqual(variant.getNextSerialNumber(), None)
 
+        # And the same for the range when serializing.
+        self.assertEqual(variant.getSerialNumberString(5), None)
+
         # This should pass, although not strictly an int field now.
         item.serial = int(n) + 1
         item.save()