fix(pipes): handle undefined value in slice

Closes #7152

modules/@angular/common/test/pipes/slice_pipe_spec.ts

@@ -41,6 +41,9 @@ export function main() {
 
     describe("transform", () => {
 
+      it('should return null if the value is null',
+         () => { expect(pipe.transform(null, [4, 2])).toBe(null); });
+
       it('should return all items after START index when START is positive and END is omitted',
          () => {
            expect(pipe.transform(list, 3)).toEqual([4, 5]);