planner: Fix load stats failure when stats file contains with `null` (#57818)#59173

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ti-chi-bot wants to merge 1 commit intopingcap:release-6.5from ti-chi-bot:cherry-pick-57818-to-release-6.5

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Commits on Jan 24, 2025

This is an automated cherry-pick of pingcap#57818

King-Dylan
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