Update MatTooltip's "message not present" test to use null/undefined …

…types outright.

We were using nullness assertions to cause these assignments to succeed, but we should allow users to set null/undefined values outright.

Right now we don't, so this update will cause the tests to fail. But this will be soon fixed.

src/material/tooltip/tooltip.spec.ts

@@ -413,12 +413,12 @@ describe('MDC-based MatTooltip', () => {
     it('should not show tooltip if message is not present or empty', () => {
       assertTooltipInstance(tooltipDirective, false);
 
-      tooltipDirective.message = undefined!;
+      tooltipDirective.message = undefined;
       fixture.detectChanges();
       tooltipDirective.show();
       assertTooltipInstance(tooltipDirective, false);
 
-      tooltipDirective.message = null!;
+      tooltipDirective.message = null;
       fixture.detectChanges();
       tooltipDirective.show();
       assertTooltipInstance(tooltipDirective, false);