LIF-977 Add safe check for userNavAnonItems

[asugar13]

Description

This PR adds optional chaining inside packages/dotcom-ui-header/src/components/top/partials.tsx. This is used as a safe check so that smoke tests can run when using dotcom-tool-kit/n-test. This issue was found while working on the this ticket: https://financialtimes.atlassian.net/browse/LIF-977
Without this change, the following error happens:

Checklist

Please read the contributing guidelines. In particular, please make sure:

• I've discussed this feature with the Platforms team
• This feature is stable, i.e. is not an ongoing experiment, temporary workaround, or hack
• My branch has been rebased onto the latest commit on main (don't merge main into your branch)

[olusoladavid]

Hi @asugar13 ,

Thanks for reporting this. Could you further describe the scenario where this was happening? Is it when you run smoke tests locally or does it happen in CI too? The TNavigationData type suggests that we don't expect this property to be sometimes optional, so would be nice to understand the root cause of the issue you're seeing.