networkd: don't stop the dhcp server if it's not configured. (#7083)

we call sd_dhcp_server_stop even if it's not configured when link is
down resulting unwanted logs.
```
Oct 10 13:41:07 nena systemd-networkd[126]: eth0: Lost carrier
Oct 10 13:41:07 nena systemd-networkd[126]: DHCP CLIENT (0x560f2dc5): STOPPED
Oct 10 13:41:07 nena systemd-networkd[126]: eth0: DHCP lease lost
Oct 10 13:41:07 nena systemd-networkd[126]: NDISC: Stopping IPv6 Router Solicitation client
Oct 10 13:41:07 nena systemd-networkd[126]: Assertion 'server' failed at ../src/libsystemd-network/sd-dhcp-server.c:256, function sd_dhcp_server_stop(). Ignoring.
```

fixes #7047

(cherry picked from commit 28464ae0)