Reverted changes on get() function

My misunderstanding of this function introduced a breaking change, which @oliverfoster caught.

A better solution is to standardise the handling of undefined or assessment 'id' values which are empty strings.  Warning messages are now output when appropriate, but the code allows an empty assessment 'id', e.g. if there is only one assessment on the course.

js/assessment.js

@@ -155,11 +155,11 @@ define([
         },
 
         _getStateByAssessmentId: function(assessmentId) {
-            if (!_.isEmpty(assessmentId)) {
-                return this._assessments._byAssessmentId[assessmentId].getState();
-            } else {
-                return this._assessments[0].getState();
+            if (assessmentId === undefined) {
+                return null;
             }
+
+            return this._assessments._byAssessmentId[assessmentId].getState();
         },
 
         _getStatesByAssessmentId: function() {
@@ -231,14 +231,18 @@ define([
             }
             this._assessments._byPageId[pageId].push(assessmentModel);
 
-            if (assessmentId) {
+            if (assessmentId !== undefined) {
+                if (assessmentId === '') {
+                    console.warn("An assessment has been registered with an empty value for 'id'");
+                }
+
                 if (!this._assessments._byAssessmentId[assessmentId]) {
                     this._assessments._byAssessmentId[assessmentId] = assessmentModel;
                 } else {
                     console.warn("An assessment with an id of '" + assessmentId + "' already exists!");
                 }
             } else {
-                console.warn("An assessment has been registered with an empty value for 'id'");
+                console.warn("An assessment has been registered with an undefined value for 'id'");
             }
 
             this._assessments.push(assessmentModel);
@@ -251,8 +255,8 @@ define([
         },
 
         get: function(id) {
-            if (_.isEmpty(id)) {
-                return this._assessments[0];
+            if (id === undefined ) {
+                return this._assessments.slice(0);
             } else {
                 return this._assessments._byAssessmentId[id];
             }