Allow undefined & null values in [matTooltip] explicitly

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);

src/material/tooltip/tooltip.ts

@@ -296,11 +296,11 @@ export class MatTooltip implements OnDestroy, AfterViewInit {
 
   /** The message to be displayed in the tooltip */
   @Input('matTooltip')
-  get message() {
+  get message(): string {
     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.

tools/public_api_guard/material/tooltip.md

@@ -76,7 +76,7 @@ export class MatTooltip implements OnDestroy, AfterViewInit {
     set hideDelay(value: NumberInput);
     _isTooltipVisible(): boolean;
     get message(): string;
-    set message(value: string);
+    set message(value: string | null | undefined);
     // (undocumented)
     ngAfterViewInit(): void;
     ngOnDestroy(): void;