dhcp_server.py: bugfix pool assignment
dhcp_server.py: bugfix pool assignment

Commit 91c3b8bdd9 ("dhcp_server.py: cleanup") did not only cleanup parts
of the code but in addition added support for DHCP failover. That support
could lead to an empty pool {} statement if the subnet declaration only had
static address assignments but no range at all.

  ---<snip>---
  dhcpd: /etc/dhcp/dhcpd.conf line 70: Pool declaration with no address
  range.
  dhcpd:         }
  dhcpd:          ^
  dhcpd: Pool declarations must always contain at least
  dhcpd: one range statement.
  ---</snip>---

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x