Update single fetch deprecation notice to also include info about null

[JakeGinnivan]

Resource routes can no longer just return null, call this out in the deprecation notice.

It just took me a minute to realise it was the null which was triggering the message.

[changeset-bot]

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[JakeGinnivan]

I'll update the tests if this is actually correct. Let me know.

[brophdawg11]

Thanks @JakeGinnivan! I made a minor update to elaborate on it also being specific to "externally accessed" resource routes. Would you mind rebasing this and pointing to dev since it's a source code change and we can get it merged for the next release?

[JakeGinnivan]

Done @brophdawg11. Thanks

Let me know if I need to squash/apply conventional commits. This started as a bit of a drive by, haha

[brophdawg11]

Looks good! No need to change any commits - I'll squash-merge the PR.

Would you mind adding a changeset as well? Should be able to just type pnpm changeset and follow the prompts for a patch change for the server-runtime package