fix empty segment cannot merge after gc and avoid write index data for empty dmfile (#4500)#4517

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ti-chi-bot wants to merge 1 commit intopingcap:release-5.4from ti-chi-bot:cherry-pick-4500-to-release-5.4

+300-74

Commits on Mar 30, 2022

This is an automated cherry-pick of pingcap#4500

lidezhu
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ti-chi-bot
committed