bgpd: fix route-distinguisher in vrf leak json cmd

bgpd: fix route-distinguisher in vrf leak json cmd

For auto configured value RD value comes as NULL,
switching back to original change will ensure to cover
for both auto and user configured RD value in JSON.

tor-11# show bgp vrf blue ipv4 unicast route-leak json
{
  "vrf":"blue",
  "afiSafi":"ipv4Unicast",
  "importFromVrfs":[
    "purple"
  ],
  "importRts":"10.10.3.11:6",
  "exportToVrfs":[
    "purple"
  ],
  "routeDistinguisher":"(null)", <<<<<
  "exportRts":"10.10.3.11:10"
}

Signed-off-by: Chirag Shah <chirag@nvidia.com>
(cherry picked from commit 892704d07f5286464728720648ad392b485a9966)