Fix rebase error after strict nulls checks

Author: jelbourn

src/lib/snack-bar/snack-bar.spec.ts

@@ -203,7 +203,7 @@ describe('MdSnackBar', () => {
       viewContainerRef: testViewContainerRef,
       verticalPosition: 'top',
     };
-    let snackBarRef = snackBar.open(simpleMessage, null, config);
+    let snackBarRef = snackBar.open(simpleMessage, '', config);
 
     viewContainerFixture.detectChanges();
     expect(snackBarRef.containerInstance.getAnimationState())