system-databases: Refactor/Improve clarity (#3178)#3353

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yikeke merged 6 commits into

pingcap:release-4.0from

ti-srebot:release-4.0-c71360707064

Jul 21, 2020

Commits on Jul 21, 2020

cherry pick #3178 to release-4.0

Null not nil
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remove 2 files
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fix conflicts
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Apply suggestions from code review
yikeke
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Update information-schema-cluster-config.md
yikeke
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Delete system-table-sql-diagnosis.md
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