zebra: handle "show evpn es-evi" a non-existent VNI

zebra: handle "show evpn es-evi" a non-existent VNI

zebra was crashing when the command was run on a non-existent VNI.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
root@torm-12:mgmt:~# net show evpn es-evi vni 16777215
VNI 16777215 doesn't exist
root@torm-12:mgmt:~# net show evpn es-evi vni 16777215 detail
VNI 16777215 doesn't exist
root@torm-12:mgmt:~# net show evpn es-evi vni 16777215 json
[
]
root@torm-12:mgmt:~# net show evpn es-evi vni 16777215 detail json
[
]
root@torm-12:mgmt:~#
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Ticket: CM-30232

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>