fix: make null rows follow schema

[jzaralim]

Description

Currently, null row values are just stored as nulls. These rows get excluded from a lot of queries from not having proper columns. This PR updates the deserializer so that it would return rows with null fields instead of just nulls.

This would fix #5633 and #5634 . However, I'm not sure if there are any side effects.

Testing done

Added QTT tests. A lot of unit tests fail - I will update these if we decide that this is actually something we want to do.

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[jzaralim]

@big-andy-coates @spena Here's what a fix for the null row problem would look like. It's not a huge change, but I'd like to know if there is anything else that should be considered.

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[vvcephei]

Closing stale PR. I'm sorry that we didn't get this done. Please resubmit as a new PR if you still want to get it in.