fix: don't flag every user as NEW when yesterday snapshot is unavailable

[vedant7007]

Problem

computeRankChanges() in scripts/sync-leaderboard.js starts with an empty previousRanks and only fills it when getYesterdaySnapshot() returns an array. That helper returns null on any error (network, GitHub API, rate-limit, missing commit). On a transient failure previousRanks stays empty, so the loop hits previousRanks[user.id] === undefined for every user and sets rankChange = "NEW" — wiping all up/down arrows. Because the result is persisted to JSON, one hiccup sticks until the next clean run.

Fix

Guard the null / non-array case: warn and leave rank changes neutral (0) instead of fabricating a board-wide "NEW". When the previous snapshot genuinely loads, behaviour is unchanged.

Notes

• Neutral 0 is the same value the existing delta-is-zero path already produces, so the frontend renders it as "no change" (not a false "NEW").
• Syntax-checked with node --check.

Closes #378

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