Gracefully handle the case in which there are no user inputs

Simple if check
Fixes e-mission/e-mission-docs#727 (comment)

www/js/survey/input-matcher.js

@@ -17,6 +17,11 @@ angular.module('emission.survey.inputmatcher', ['emission.plugin.logger'])
     // If there is only one item in the list, return it.
     // This make it compatible when fake list is given (for Enketo Survey).
     // As well as optimize the performance.
+    if (userInputList === undefined) {
+        Logger.log("In getUserInputForTrip, no user input, returning []");
+        return undefined;
+    }
+
     if (userInputList.length === 1) {
       return userInputList[0];
     }