Allow null and undefined values for [matTooltip] explicitly.

We have tests that already set these values (but used to use nullness assertions to overcome the build failures) and protect against it by treating these values as empty strings (and thus we hide the tooltip), but we don't allow anyone who uses templates with strict type checking to set optional values unless they add their own nullness assertion.

src/material/tooltip/tooltip.ts

@@ -300,7 +300,7 @@ export class MatTooltip implements OnDestroy, AfterViewInit {
     return this._message;
   }
 
-  set message(value: string) {
+  set message(value: string | null | undefined) {
     this._ariaDescriber.removeDescription(this._elementRef.nativeElement, this._message, 'tooltip');
 
     // If the message is not a string (e.g. number), convert it to a string and trim it.