zebra:return empty dict when evpn is disabled

zebra:return empty dict when evpn is disabled

"show evpn json" returns nothing when evpn is disabled.

Code has been fixed to return {} when evpn is disabled or no entry
available.

Before Fix:-
```
cumulus@r2:mgmt:~$ sudo vtysh -c "show evpn json"
cumulus@r2:mgmt:~$
```

After Fix:-
```
cumulus@r1:mgmt:~$ sudo vtysh -c "show evpn json"
{
}
cumulus@r1:mgmt:~$
```

Ticket:#3417955

Issue:3417955

Testing: UT done

Signed-off-by: Chirag Shah <chirag@nvidia.com>
Signed-off-by: Sindhu Parvathi Gopinathan <sgopinathan@nvidia.com>