Fixes GITLABCOM-PYW8

What does this MR do and why?

Fixes https://sentry.gitlab.net/gitlab/gitlabcom/issues/3358139/ and related errors, that happens when Oj returns nil when parsing a notebook.

How to set up and validate locally

This is a corner case that is hard to reproduce manually, but the new test replicates the error.

MR acceptance checklist

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