fix: change access signature to Astro.session

[ematipico]

Description (required)

This PR fixes the call of Astro.session and ctx.session. The session object might be undefined, so we need the ? to gracefully handle its access.

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Tracked Files

File	Note
en/guides/integrations-guide/cloudflare.mdx	Source changed, localizations will be marked as outdated.
en/guides/sessions.mdx	Source changed, localizations will be marked as outdated.
en/reference/api-reference.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[yanthomasdev]

LGTM! Thanks!