Allow action URIs to be retrieved without errors if they are missing

thingActionUrl may now be told to return `undefined` insstead of throwing an error if the action is missing.

webapp/src/main.js

@@ -71,12 +71,12 @@ Vue.mixin({
       }
       await axios.put(url, value);
     },
-    thingActionUrl(thing, action) {
+    thingActionUrl(thing, action, allow_missing=false) {
       let url = this.$store.getters["wot/thingActionUrl"](
         thing,
         action,
         "invokeaction",
-        false
+        allow_missing
       );
       return url;
     },

webapp/src/wot-client.js

@@ -120,6 +120,7 @@ export const wotStoreModule = {
 
 export function findFormHref(affordance, op) {
   // Find the form in the affordance that matches the given operation type
+  if (affordance == undefined) return undefined;
   let forms = affordance.forms;
   let matchingForm = forms.find(f => f.op == op || f.op.includes(op));
   if (matchingForm == undefined) return undefined;