fix: crash when page not found & using multiVersion

[Timeless0911]

Summary

same as #987

When using multiVersion, the useNav hook defaults to 'default' version when version from useVersion is empty. We should first check for defaultVersion in siteData.multiVersion.default before going with default as a key for nav.

Edit: as this happens on 404 page only, it's better to supply defaultVersion from siteData.multiVersion.default in initPageData, similar as it is done for lang. This ensured that:

• - page doesn't crash in multiVersion setup
• - when using search.versioned, fetching search index doesn't fail, and fetches default version index
• - default version is displayed on 404 page version dropdown

Related Issue

closes #986

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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