use optional fields for refetch variables

[tsnobip]

Before this PR, untouched optional variables were set as null instead of not being defined. This is now fixed.

To be used with zth/relay#32

[zth]

This is great! But, if we're to merge this, I would really really like the test suite for refetch variables to be extended thoroughly to handle all relevant cases for how refetch variables is supposed to behave.

Here's the relevant docs section: https://relay.dev/docs/next/api-reference/use-refetchable-fragment/#return-value
Probably easy work for an LLM to extend the test suite for this to provide 100% expected behavior coverage (the correct variables change depending on what you send, and the expected thing is dispatched to the server, etc).