sdn: evpn: allow empty primary exit node in zone form

its broken since the change in semantics of the PUT endpoint [1]

[1] https://git.proxmox.com/?p=pve-network.git;a=commit;h=3e3cafabaf955d53c4c2d4e346bf5c3a5c6d1852

Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>
Originally-by: Alexandre Derumier <aderumier@odiso.com>

www/manager6/sdn/zones/EvpnEdit.js

@@ -54,6 +54,8 @@ Ext.define('PVE.sdn.zones.EvpnInputPanel', {
 		fieldLabel: gettext('Primary Exit Node'),
 		multiSelect: false,
 		autoSelect: false,
+		skipEmptyText: true,
+		deleteEmpty: !me.isCreate,
 	    },
 	    {
 		xtype: 'proxmoxcheckbox',