Allow undefined & null values in [matTooltip] explicitly (#29397)

* fix(material/tooltip): 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.

* fix(material/tooltip): 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.

* 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.

* ci: Approve new API goldens for tooltip

---------

Co-authored-by: Miles Malerba <mmalerba@google.com>

Author: Eyas

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;