Fixed bug where a blank card is displayed

on-tap is attached to both the chip-container and the card.
To know which card gets displayed, the transition function called
by on-tap assigns the selectedAlbum variable the value of the target's
templateInstance.model.item, which is null if the user clicks on any
area which is inside chip-container but outside a chip. This results
to a blank card being displayed.

The simplest fix for this is to check if selectedAlbum is null before
setting the page flag to 1.

demos/music.html

@@ -156,7 +156,9 @@
       transition: function(e) {
         if (this.page === 0) {
           this.selectedAlbum = e.target.templateInstance.model.item;
-          this.page = 1;
+          if(this.selectedAlbum != null) {
+            this.page = 1;
+          }
         } else {
           this.page = 0;
         }