bgpd: fix blank line in running-config with bmp listener cmd

bgpd: fix blank line in running-config with bmp listener cmd

An extra blank line is added in show running-config with BMP:

> ubuntu2204hwe(config)# router bgp 65500
> ubuntu2204hwe(config-router)# bmp targets tgt
> ubuntu2204hwe(config-bgp-bmp)# bmp monitor ipv4 unicast pre-policy
> ubuntu2204hwe(config-bgp-bmp)# bmp listener 192.0.2.100 port 44
> ubuntu2204hwe(config-bgp-bmp)# do show running-config
>
> router bgp 65500
> [..]
>  bmp targets tgt
>   bmp monitor ipv4 unicast pre-policy
>                                       <-- blank line
>   bmp listener 192.0.2.100 port 44
>  exit

Remove the blank line.

Fixes: ed18356f1f2d ("bgpd/bmp: BMP implementation")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>