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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[gemini-code-assist]

Code Review

This pull request updates the Grafana dashboard configuration for TiCDC by changing the nullPointMode from "null" to "null as zero" in the metrics definition. I have no feedback to provide as there were no review comments to evaluate.