bgpd: fix json output for show cmd for evpn routes

bgpd: fix json output for show cmd for evpn routes

Fix json output where show command displayed
"Route Distinguisher: " string in json output because
NULL json object passed to bgp_evpn_show_route_rd_header()

Testing Done:
Route Distinguisher: ip 27.0.0.11:3
Route Distinguisher: ip 27.0.0.11:4
Route Distinguisher: ip 27.0.0.16:3
Route Distinguisher: ip 27.0.0.16:5
Route Distinguisher: ip 27.0.0.16:9
{
  "27.0.0.11:3":{
    "rd":"27.0.0.11:3",
    "[2]:[0]:[48]:[00:02:00:00:00:02]":{
      "prefix":"[2]:[0]:[48]:[00:02:00:00:00:02]",
      "prefixLen":288,
 ...
}

Post fix:
{
  "27.0.0.11:3":{
    "rd":"27.0.0.11:3",
    "[2]:[0]:[48]:[00:02:00:00:00:02]":{
      "prefix":"[2]:[0]:[48]:[00:02:00:00:00:02]",
      "prefixLen":288,
  ...
}
Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>