Display message when no changes are available

Message is kept unspecific since it can be shown because of search
query with no matches or dashboard group with no items.

Change-Id: I02020aa51651e6a2c1c8a72908ce2fc5bd77bd73

polygerrit-ui/app/elements/gr-change-list.html

@@ -49,6 +49,9 @@ limitations under the License.
       <template is="dom-if" if="[[_groupTitle(groupIndex)]]">
         <div class="groupHeader">[[_groupTitle(groupIndex)]]</div>
       </template>
+      <template is="dom-if" if="[[!changeGroup.length]]">
+        <div class="noChanges">No changes</div>
+      </template>
       <template is="dom-repeat" items="[[changeGroup]]" as="change">
         <gr-change-list-item
             selected$="[[_computeItemSelected(index, groupIndex, selectedIndex)]]"

polygerrit-ui/app/styles/gr-change-list-styles.html

@@ -29,6 +29,10 @@ limitations under the License.
         background-color: #ddd;
         flex-shrink: 0;
       }
+      .noChanges {
+        border-bottom: 1px solid #eee;
+        padding: .3em .5em;
+      }
       .keyboard,
       .star {
         align-items: center;

polygerrit-ui/app/test/gr-change-list-test.html

@@ -136,6 +136,26 @@ limitations under the License.
       assert.isFalse(elementItems[2].hasAttribute('unreviewed'));
     });
 
+    test('no changes', function() {
+      element.changes = [];
+      flushAsynchronousOperations();
+      var listItems = Polymer.dom(element.root).querySelectorAll(
+          'gr-change-list-item');
+      assert.equal(listItems.length, 0);
+      var noChangesMsg = Polymer.dom(element.root).querySelector('.noChanges');
+      assert.ok(noChangesMsg);
+    });
+
+    test('empty groups', function() {
+      element.groups = [[], []];
+      flushAsynchronousOperations();
+      var listItems = Polymer.dom(element.root).querySelectorAll(
+          'gr-change-list-item');
+      assert.equal(listItems.length, 0);
+      var noChangesMsg = Polymer.dom(element.root).querySelectorAll(
+          '.noChanges');
+      assert.equal(noChangesMsg.length, 2);
+    });
   });
 
   suite('gr-change-list groups', function() {