bgpd: fix incorrect json output in bgp_show_table_rd()

bgpd: fix incorrect json output in bgp_show_table_rd()

'{}' extra output may present JSON output from bgp_show_table_rd() when
no prefix are seen.

> {
>  "vrfId": 0,
>  "vrfName": "default",
>  "tableVersion": 0,
>  "routerId": "1.1.1.1",
>  "defaultLocPrf": 100,
>  "localAS": 65500,
>  "routes": {  "routeDistinguishers" : { "444:1" : {  }  }  }  }
> {}

Do not output '{}' when bgp_show_table() returns a valid JSON.
Tested without rd in config, bgp_show_table() returns nothing and the
JSON output is only '{}'

Fixes: 0224b3296c ("bgpd: Print empty JSON `{}` if no entries under `show bgp ipv4 vpn json`")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>