network_config.js: Avoid assert when vpnType_ is undefined

With Polymer2, observers may be called with undefined parameters.
updateVpnType_() which predates the migration was assuming that
vpnType_ was defined.

Bug: 1068533
Change-Id: I6583d7ec72b591dcc4dcaa75a922f5de36373655
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2139442Reviewed-by: James Vecore <vecore@google.com>
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757210}

ui/webui/resources/cr_components/chromeos/network/network_config.js

@@ -990,7 +990,7 @@ Polymer({
 
   /** @private */
   updateVpnType_() {
-    if (this.configProperties_ === undefined) {
+    if (this.configProperties_ === undefined || this.vpnType_ === undefined) {
       return;
     }