lib: show "(null)" for empty IP address

lib: show "(null)" for empty IP address

Use "(null)" for empty IP address.

One example in `bgp_zebra_send_remote_macip()` to install mac:

Before:
```
2023/01/18 02:09:09 BGP: [SCHS5-AK960] Tx ADD MACIP, VNI 200 MAC 06:6b:7c:db:83:72 IP  flags 0x0 seq 0 remote VTEP 88.88.88.88 esi -
```

After:
```
2023/01/18 20:19:57 BGP: [SCHS5-AK960] Tx ADD MACIP, VNI 200 MAC 06:6b:7c:db:83:72 IP (null) flags 0x0 seq 0 remote VTEP 88.88.88.88 esi -
```

Signed-off-by: anlan_cs <vic.lan@pica8.com>