bgpd: fix proper json format for unicast statistics for non exists vrf

bgpd: fix proper json format for unicast statistics for non exists vrf

Ticket: #4060069

show bgp vrf afi unicast statistics json output is not return in json
format for non exists vrf.

Fix:
Json output is formatted for non exists vrf cases.

Command supported:

```
show bgp vrf <VRFNAME> ipv4/ipv6 unicast statistics json
show bgp vrf <VRFNAME> l2vpn evpn statistics json
```

Before Fix:

```
leaf11#
leaf11# show bgp vrf test ipv4 unicast statistics json
View/Vrf test is unknown
leaf11#
leaf11#
leaf11# show bgp vrf test ipv6 unicast statistics json
View/Vrf test is unknown
leaf11#
leaf11#
leaf11# show bgp vrf default1 l2vpn evpn statistics json
View/Vrf default1 is unknown
leaf11#

```

After Fix:

```
leaf11#
leaf11# show bgp vrf test ipv4 unicast statistics json
{
  "warning":"View/Vrf is unknown"
}
leaf11#
leaf11#
leaf11# show bgp vrf test ipv6 unicast statistics json
{
  "warning":"View/Vrf is unknown"
}
leaf11#
leaf11# show bgp vrf default1 l2vpn evpn statistics json
{
  "warning":"View/Vrf is unknown"
}
leaf11#
```

Ticket: #4060069

Signed-off-by: Sindhu Parvathi Gopinathan's <sgopinathan@nvidia.com>