fix(tooltip): empty tooltips should not show up.

Fixes #8021.

src/components/tooltip/tooltip.js

@@ -274,6 +274,9 @@ function MdTooltipDirective($timeout, $window, $$rAF, $document, $mdUtil, $mdThe
  }
 
  function showTooltip() {
+ // Do not show the tooltip if the text is empty.
+ if (!element[0].textContent.trim()) return;
+
  // Insert the element and position at top left, so we can get the position
  // and check if we should display it
  element.css({top: 0, left: 0});

src/components/tooltip/tooltip.spec.js

@@ -165,6 +165,33 @@ describe('<md-tooltip> directive', function() {
  expect($rootScope.testModel.isVisible).toBe(false);
  });
 
+ it('should not show when the text is empty', function() {
+
+ expect(findTooltip().length).toBe(0);
+
+ buildTooltip(
+ '<md-button>' +
+ 'Hello' +
+ '<md-tooltip md-visible="testModel.isVisible">{{ textContent }} </md-tooltip>' +
+ '</md-button>'
+ );
+
+ showTooltip(true);
+
+ expect(findTooltip().length).toBe(0);
+
+ $rootScope.textContent = 'Tooltip';
+ $rootScope.$apply();
+
+ // Trigger a change on the model, otherwise the tooltip component can't detect the
+ // change.
+ showTooltip(false);
+ showTooltip(true);
+
+ expect(findTooltip().length).toBe(1);
+ expect(findTooltip().hasClass('_md-show')).toBe(true);
+ });
+
  it('should set visible on focus and blur', function() {
  buildTooltip(
  '<md-button>' +