zebra:returns empty dict when evpn is disabled II

zebra:returns empty dict when evpn is disabled II

This is addressing remaining places returning
empty dict, earlier PR-13214 addressed few places.

Code has been changed to return {} for all the evpn clis
when evpn is disabled or no entry available.

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