Fix new tree deletion

[zoriya]

Fixes #2836

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Before the change?

• When creating a tree with a null content & sha, we got this error from github: Must supply either tree.sha or tree.content. Request will be rejected if both are present
• Since null are not serialized, gh received neither a sha nor a content value in the tree and that caused the error.

After the change?

• Actually serialize the null value of the sha to allow deletion of tree items.

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

I made an integration test but running it is a bit of a pain, I noticed there's no CI on my fork and i am opening this PR to get CI runs x)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[zoriya]

Note that i also tested this in one of our project and i can confirm it works.

[github-actions]

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[zoriya]

We've been using this PR in production for almost a year, would be great to have this merged!