Fix an error on browser console when Last.fm has no tags for the entity

There was an error message shown on the browser console when the
Last.fm info was viewed on the details pane and the data for the viewed
artist/album/track happened to contain not a single `tag` property.
Other than the message, this caused no actual harm.

js/app/controllers/sidebar/sidebarcontroller.js

@@ -132,7 +132,9 @@ angular.module('Music').controller('SidebarController', [
 		});
 
 		$scope.formatLastfmTags = function(tags) {
-			// Last.fm returns individual JSON object in place of array in case there is just one item
+			// Last.fm returns individual JSON object in place of array in case there is just one item.
+			// In case there are none, the `tags` is undefined.
+			tags = tags || [];
 			if (!Array.isArray(tags)) {
 				tags = [tags];
 			}
@@ -149,6 +151,8 @@ angular.module('Music').controller('SidebarController', [
 
 		$scope.formatLinkList = function(linkArray) {
 			// Last.fm returns individual JSON object in place of array in case there is just one item
+			// In case there are none, the `linkArray` is undefined.
+			linkArray = linkArray || [];
 			if (!Array.isArray(linkArray)) {
 				linkArray = [linkArray];
 			}