Always send explicit lists for userLogins, botLogins, and teamSlugs
in RequestReviewsByLogin mutation, even when empty. Previously, empty
slices were omitted due to omitempty JSON behavior and len > 0 checks,
which prevented clearing all reviewers when using replace mode.
Empty slices are harmless no-ops in union mode, so we now send them
unconditionally for simpler logic.
Add test to verify the mutation receives empty slices when all
reviewers are removed.
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