fixup! refactor(cdk-experimental/ui-patterns): track focus by item not index

Author: wagnermaciel

src/cdk-experimental/ui-patterns/behaviors/list/list.spec.ts

@@ -23,7 +23,7 @@ describe('List Behavior', () => {
   function getList<V>(inputs: Partial<TestInputs<V>> & Pick<TestInputs<V>, 'items'>): TestList<V> {
     return new List({
       value: inputs.value ?? signal([]),
-      activeItem: signal(null),
+      activeItem: signal(undefined),
       typeaheadDelay: inputs.typeaheadDelay ?? signal(0.5),
       wrap: inputs.wrap ?? signal(true),
       disabled: inputs.disabled ?? signal(false),