fix(GlListbox): Allows listbox to accept null values as defaults

What does this MR do and why?

This MR improves the compatibility with !5047 (merged) by allowing null values to be set as defaults while continuing to support the fix at #2144 (closed).

How?

In #2144 (closed) we reported a bug in which providing a null value would stop the keyboard navigation from working. This was only part of the story.

With this fix we ensure the first item is highlighted even if the value is null, or something else that is not in the list.

References

Screenshots or screen recordings

Selecting the null value failed:

Before

After

Note I also tried this locally in my GDK using yalc and it works well:

How to set up and validate locally

1. Visit http://localhost:9002/?path=/story/base-dropdown-collapsible-listbox--default and test the options.

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