types: fix a bug in casting str2str when `union` (#37242)#37353

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ti-srebot wants to merge 1 commit intopingcap:release-4.0from ti-srebot:release-4.0-25dda978ff9f

+1,288-1

Commits on Aug 24, 2022

cherry pick pingcap#37242 to release-4.0

CbcWestwolf
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ti-srebot
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