fix(tabs): properly blank activedescendant attr when no tabs exist (#9907)

`aria-activedescendant` should be blank if no tabs could be found

* Add null check when searching for currently focused tab

Fixes #9279

Author: clshortfuse

src/components/tabs/js/tabsController.js

@@ -63,6 +63,7 @@ function MdTabsController ($scope, $element, $window, $mdConstant, $mdTabInkRipp
   ctrl.getTabElementIndex = getTabElementIndex;
   ctrl.updateInkBarStyles = $mdUtil.debounce(updateInkBarStyles, 100);
   ctrl.updateTabOrder     = $mdUtil.debounce(updateTabOrder, 100);
+  ctrl.getFocusedTabId    = getFocusedTabId;
 
   init();
 
@@ -339,7 +340,7 @@ function MdTabsController ($scope, $element, $window, $mdConstant, $mdTabInkRipp
       ctrl.offsetLeft = fixOffset(tab.offsetLeft);
     } else {
       /**
-       * Canvas width *smaller* than tab width: positioning at the *end* of current tab to let 
+       * Canvas width *smaller* than tab width: positioning at the *end* of current tab to let
        * pagination "for loop" to proceed correctly on next tab when nextPage() is called again
        */
       ctrl.offsetLeft = fixOffset(tab.offsetLeft + (tab.offsetWidth - viewportWidth + 1));
@@ -362,10 +363,10 @@ function MdTabsController ($scope, $element, $window, $mdConstant, $mdTabInkRipp
       ctrl.offsetLeft = fixOffset(tab.offsetLeft + tab.offsetWidth - elements.canvas.clientWidth);
     } else {
       /**
-       * Canvas width *smaller* than tab width: positioning at the *beginning* of current tab to let 
+       * Canvas width *smaller* than tab width: positioning at the *beginning* of current tab to let
        * pagination "for loop" to break correctly on previous tab when previousPage() is called again
        */
-      ctrl.offsetLeft = fixOffset(tab.offsetLeft);  
+      ctrl.offsetLeft = fixOffset(tab.offsetLeft);
     }
   }
 
@@ -494,6 +495,17 @@ function MdTabsController ($scope, $element, $window, $mdConstant, $mdTabInkRipp
         ctrl.offsetLeft;
   }
 
+  /**
+   * Returns currently focused tab item's element ID
+   */
+  function getFocusedTabId() {
+    var focusedTab = ctrl.tabs[ctrl.focusIndex];
+    if (!focusedTab || !focusedTab.id) {
+      return null;
+    }
+    return 'tab-item-' + focusedTab.id;
+  }
+
   /**
    * Determines if the UI should stretch the tabs to fill the available space.
    * @returns {*}

src/components/tabs/js/tabsDirective.js

@@ -132,7 +132,7 @@ function MdTabs ($$mdSvgRegistry) {
           '</md-next-button> ' +
           '<md-tabs-canvas ' +
               'tabindex="{{ $mdTabsCtrl.hasFocus ? -1 : 0 }}" ' +
-              'aria-activedescendant="tab-item-{{$mdTabsCtrl.tabs[$mdTabsCtrl.focusIndex].id}}" ' +
+              'aria-activedescendant="{{$mdTabsCtrl.getFocusedTabId()}}" ' +
               'ng-focus="$mdTabsCtrl.redirectFocus()" ' +
               'ng-class="{ ' +
                   '\'md-paginated\': $mdTabsCtrl.shouldPaginate, ' +