evpn: Fix evpn_vni column condition

The following SDK patch [1] now always has the evpn_vni router
attribute defined but can be None. Even when it's None, the column
gets populated. The original patch with change-id
I8747f3cb753a115e7955b0a0ea136431fdade0dd missed this condition.

Related-Bug: #2144617

[1] https://review.opendev.org/c/openstack/openstacksdk/+/991685

Change-Id: I0dffaeb4c3315c0ed44881135b024460cc4538b7
Signed-off-by: Jakub Libosvar <jlibosva@redhat.com>

Author: cubeek

openstackclient/network/v2/router.py

@@ -88,7 +88,7 @@ def _get_columns(
     if item.is_distributed is None:
         hidden_columns.append('is_distributed')
         column_map.pop('is_distributed')
-    if not hasattr(item, 'evpn_vni'):
+    if not hasattr(item, 'evpn_vni') or item.evpn_vni is None:
         hidden_columns.append('evpn_vni')
     return utils.get_osc_show_columns_for_sdk_resource(
         item, column_map, hidden_columns