header: Make heading optional and conditionally render

[stowball]

Another government department does not need to use the heading prop, but this unfortunately creates an accessibility violation of rendering an empty link.

This PR makes heading optional, and updates the conditional rendering as necessary.

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Checklist

Preflight

• Prefix the PR title with the slug of the package or component - e.g. accordion: Updated padding or docs: Updated header links
• Describe the changes clearly in the PR description
• Read and check your code before tagging someone for review
• Create a changeset file by running yarn changeset. Learn more about change management.

Testing

• Manually test component in various modern browsers at various sizes (use Browserstack)
• Manually test component in various devices (phone, tablet, desktop)
• Manually test component using a keyboard
• Manually test component using a screen reader
• Manually tested in dark mode
• Component meets Web Content Accessibility Guidelines (WCAG) 2.1 standards
• Add any necessary unit tests (HTML validation, snapshots etc)
• Run yarn test to ensure tests are passing. If required, run yarn test -u to update any generated snapshots.

Documentation

• Create or update documentation on the website
• Create or update stories for Storybook
• Create or update stories for Playroom snippets
• Verify hard-coded anchor links and hashes are correct. Check existing links when renaming pages and headings.

[changeset-bot]

🦋 Changeset detected

Latest commit: 3d0f145

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

This PR includes changesets to release 1 package

Name	Type
@ag.ds-next/react	Minor

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud