fix(common): correct typing and implementation of SlicePipe (#37447)

Even in the overloads, state that it can accept `null` and
`undefined`, in order to ensure easy composition with `async`.

Additionally, change the implementation to return `null` on an
`undefined` input, for consistency with other pipes.

BREAKING CHANGE:
The `slice` pipe now returns `null` for the `undefined` input value,
which is consistent with the behavior of most pipes. If you rely on
`undefined` being the result in that case, you now need to check for it
explicitly.

PR Close #37447

packages/common/test/pipes/slice_pipe_spec.ts

@@ -47,8 +47,8 @@ import {expect} from '@angular/platform-browser/testing/src/matchers';
         expect(pipe.transform(null, 1)).toBe(null);
       });
 
-      it('should return undefined if the value is undefined', () => {
-        expect(pipe.transform(undefined, 1)).toBe(undefined);
+      it('should return null if the value is undefined', () => {
+        expect(pipe.transform(undefined, 1)).toBe(null);
       });
 
       it('should return all items after START index when START is positive and END is omitted',