[SelectPanel] Add checks for existence of filter input

[jamieshark]

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What are you trying to accomplish?

Previously we were encountering an error when we were trying to access the filterInputTextFieldElement before it was initialized. This change ensures that we are bailing out if the element does not exist. Also updates the playground instance to allow for show_filter param to test changes.

Integration

N/A

List the issues that this change affects.

• Unblocks https://github.com/github/github/pull/338989
• Follow up to https://github.com/github/primer/issues/3707

Risk Assessment

• Low risk the change is small, highly observable, and easily rolled back.
• Medium risk changes that are isolated, reduced in scope or could impact few users. The change will not impact library availability.
• High risk changes are those that could impact customers and SLOs, low or no test coverage, low observability, or slow to rollback.

What approach did you choose and why?

Anything you want to highlight for special attention from reviewers?

Accessibility

• Fixes axe scan violation - This change fixes an existing axe scan violation.
• No new axe scan violation - This change does not introduce any new axe scan violations.
• New axe violation - This change introduces a new axe scan violation. Please describe why the violation cannot be resolved below.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Lookbook)
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

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[changeset-bot]

🦋 Changeset detected

Latest commit: 03b7d75

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/view-components	Patch

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[camertron]

Thank you!