grafana: render stacked nulls as zero

[nizheming]

Summary

• set stacked Grafana graph panels to render null points as zero
• avoid misleading stacked visualizations when a series has data gaps

Test

• parsed dashboard JSON files
• verified every stack: true graph panel has nullPointMode: "null as zero"

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[ti-chi-bot]

Welcome @nizheming! It looks like this is your first PR to pingcap/monitoring 🎉

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