fix: only require at least 1 field to be present in a collection update

[jackwilsdon]

Change Summary

The collection update endpoint only requires that at least one property is present - "fields" is not strictly required (e.g. when doing metadata updates).

https://github.com/typesense/typesense/blob/1c5804194175210b3fbdd4972ba3cff2bbdf85cf/src/core_api.cpp#L301-L367

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