fix(tooltip): don't show tooltip if message is empty or not present (#2081)

Closes #2078

src/lib/tooltip/tooltip.spec.ts

@@ -74,6 +74,30 @@ describe('MdTooltip', () => {
       expect(tooltipDirective._tooltipInstance).toBeNull();
     }));
 
+    it('should not show tooltip if message is not present or empty', fakeAsync(() => {
+      expect(tooltipDirective._tooltipInstance).toBeUndefined();
+
+      tooltipDirective.message = undefined;
+      fixture.detectChanges();
+      tooltipDirective.show();
+      expect(tooltipDirective._tooltipInstance).toBeUndefined();
+
+      tooltipDirective.message = null;
+      fixture.detectChanges();
+      tooltipDirective.show();
+      expect(tooltipDirective._tooltipInstance).toBeUndefined();
+
+      tooltipDirective.message = '';
+      fixture.detectChanges();
+      tooltipDirective.show();
+      expect(tooltipDirective._tooltipInstance).toBeUndefined();
+
+      tooltipDirective.message = '   ';
+      fixture.detectChanges();
+      tooltipDirective.show();
+      expect(tooltipDirective._tooltipInstance).toBeUndefined();
+    }));
+
     it('should not follow through with hide if show is called after', fakeAsync(() => {
       tooltipDirective.show();
       expect(tooltipDirective._isTooltipVisible()).toBe(true);

src/lib/tooltip/tooltip.ts

@@ -98,6 +98,10 @@ export class MdTooltip {
 
   /** Shows the tooltip */
   show(): void {
+    if (!this._message || !this._message.trim()) {
+      return;
+    }
+
     if (!this._tooltipInstance) {
       this._createTooltip();
     }