fix(common): correct and simplify typing of AsyncPipe (#37447)

`AsyncPipe.transform` will never return `undefined`, even when passed
`undefined` in input, in contrast with what was declared in the
overloads.

Additionally the "actual" method signature can be updated to match the
most generic case, since the implementation does not rely on wrappers
anymore.

BREAKING CHANGE:
The async pipe no longer claims to return `undefined` for an input that
was typed as `undefined`. Note that the code actually returned `null` on
`undefined` inputs. In the unlikely case you were relying on this,
please fix the typing of the consumers of the pipe output.

PR Close #37447

packages/language-service/test/definitions_spec.ts

@@ -251,7 +251,7 @@ describe('definitions', () => {
     expect(textSpan).toEqual(marker);
 
     expect(definitions).toBeDefined();
-    expect(definitions!.length).toBe(4);
+    expect(definitions!.length).toBe(3);
 
     const refFileName = '/node_modules/@angular/common/common.d.ts';
     for (const def of definitions!) {