fix(material/tooltip): Also explicitly define the type of MatTooltip.message getter as always a string.

We do this since the underlying `._message` type is a string, the message setter can accept null and undefined values, but will convert them to strings before saving to `_message`, so we can always assume someone _reading_ the value will see a plain string.

Author: Eyas

src/material/tooltip/tooltip.ts

@@ -296,7 +296,7 @@ export class MatTooltip implements OnDestroy, AfterViewInit {
 
   /** The message to be displayed in the tooltip */
   @Input('matTooltip')
-  get message() {
+  get message(): string {
     return this._message;
   }