Work around unpopulated hwsku in db_migrator.py

Handle the case where hwsku is None.
See sonic-net/sonic-buildimage#12083 and sonic-net/sonic-buildimage#11416

scripts/db_migrator.py

@@ -890,7 +890,7 @@ def common_migration_ops(self):
         # removed together with calling to migrate_copp_table function.
         if self.asic_type != "mellanox":
             self.migrate_copp_table()
-        if self.asic_type == "broadcom" and 'Force10-S6100' in self.hwsku:            
+        if self.asic_type == "broadcom" and self.hwsku and 'Force10-S6100' in self.hwsku:
             self.migrate_mgmt_ports_on_s6100()
         else:
             log.log_notice("Asic Type: {}, Hwsku: {}".format(self.asic_type, self.hwsku))