fix: Send null for uploaded files in multipart operations

[knd775]

JSON.stringify serializes a File into {}, which violates the multipart upload spec and breaks requests to some strict servers (in our case, Hot Chocolate starting with version 16)

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[changeset-bot]

🦋 Changeset detected

Latest commit: 08642bc

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
houdini	Patch
houdini-core	Patch

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