sharpd: fix deleting nhid when suppressing nexthop from nh group

sharpd: fix deleting nhid when suppressing nexthop from nh group

When removing the last nexthop from a nexthop-group, the nexthop
group remains in the zebra contexts:

> ubuntu2204(config)# nexthop-group gdgd
> 2023/11/23 14:06:36 SHARP: [Q5NBA-GN1BG] NHG ID assigned: 179687502
> ubuntu2204(config-nh-group)# nexthop 192.0.2.7 loop1
> ubuntu2204(config-nh-group)# 2023/11/23 14:06:38 ZEBRA: [VNMVB-91G3G] _netlink_nexthop_build_group: ID (179687502): group 338
> 2023/11/23 14:06:38 ZEBRA: [R43C6-KYHWT] netlink_nexthop_msg_encode: RTM_NEWNEXTHOP, id=179687502
> 2023/11/23 14:06:38 ZEBRA: [HYEHE-CQZ9G] nl_batch_send: netlink-dp (NS 0), batch size=44, msg cnt=1
> 2023/11/23 14:06:38 SHARP: [JWRCN-N9K90] Installed nhg 179687502
>
> ubuntu2204(config-nh-group)# no nexthop 192.0.2.7 loop1
> 2023/11/23 14:06:47 SHARP: [Y2G2F-ZTW6M] nhg_add: nhg 179687502 not sent: no valid nexthops
> ubuntu2204(config-nh-group)# do show nexthop-group rib 179687502
> ID: 179687502 (sharp)
>      RefCnt: 1
>      Uptime: 00:00:23
>      VRF: default
>      Valid, Installed
>      Depends: (338)
>            via 192.0.2.7, loop1 (vrf default), weight 1

Fix this by sending an NHG_DEL message when no nexthops are attached,
and when the id was already installed.

Fixes: 5a9c0931aa95 ("sharpd: don't send invalid nexthop-groups to zebra")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>