Filtering non existing item in p-dropdown [filter=true, group=true] and pressing key down generates error: Dropdown.html:25 ERROR TypeError: Cannot read property '0' of null

[gintskrastins]

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Plunkr Case (Bug Reports)
https://stackblitz.com/edit/github-qrz9jq?file=package.json

Current behavior
After filtring non-existing item and pressing the key down occurs error:

  Dropdown.html:25 ERROR TypeError: Cannot read property '0' of null
    at Dropdown.onKeydown (dropdown.ts:861)
    at Object.eval [as handleEvent] (Dropdown.html:26)
    at handleEvent (view.ts:138)
    at callWithDebugContext (services.ts:641)
    at Object.debugHandleEvent [as handleEvent] (services.ts:381)
    at dispatchEvent (util.ts:135)
    at eval (element.ts:208)
    at HTMLInputElement.eval (dom_renderer.ts:66)
    at ZoneDelegate.invokeTask (zone.js:434)
    at Object.onInvokeTask (ng_zone.ts:390)

Expected behavior
Error is not expected

Minimal reproduction of the problem with instructions

• Open https://github-qrz9jq.stackblitz.io

• Filter for "abc" and press key down

What is the motivation / use case for changing the behavior?
Error is not expected.

This is identical reproduction scenario to #10325 only reproduced on latest version.

Please tell us about your environment:

• Angular version: 12.0.1

• PrimeNG version: 12.0.0-rc.1

• Browser: [Chrome 91.0.4472.77 | Firefox 89.0 (64-bit)]