Fix refetch empty array

[zth]

Note

Low Risk
Small, localized change to variable-sanitization logic with added unit coverage; primary risk is subtle regressions in option/sentinel detection for refetch variables.

Overview
Fixes a bug in internal_removeUndefinedAndConvertNullsRaw where detecting Some(None) via value == Some(None) could mis-handle values like empty arrays during refetch variable cleaning.

The function now explicitly detects ReScript’s Some(None) sentinel (BS_PRIVATE_NESTED_SOME_NONE) and only converts that case to null, while continuing to drop None/undefined fields and leaving arrays (including empty arrays) intact. Adds a Jest test covering empty-array preservation and basic dropping/keeping behavior, and documents the fix in CHANGELOG.md.

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