[Fix](Variant) fix variant schema change may cause invalid block schema and write missing blocks #36317

[eldenmoon]

…ma and write missing blocks (#36317)

buffering block during schema change should merge first and then flush, otherwise may lost current block

Proposed changes

Issue Number: close #xxx

[doris-robot]

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[eldenmoon]

run buildall

[eldenmoon]

run buildall

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[doris-robot]

TeamCity be ut coverage result:
Function Coverage: 36.30% (9133/25158)
Line Coverage: 27.79% (74259/267225)
Region Coverage: 26.72% (38311/143379)
Branch Coverage: 23.41% (19405/82896)
Coverage Report: http://coverage.selectdb-in.cc/coverage/c7c07971af032bf6b91256a1a9af9afd1bad379d_c7c07971af032bf6b91256a1a9af9afd1bad379d/report/index.html

[doris-robot]

TeamCity be ut coverage result:
Function Coverage: 36.30% (9133/25158)
Line Coverage: 27.79% (74261/267225)
Region Coverage: 26.73% (38321/143379)
Branch Coverage: 23.41% (19408/82896)
Coverage Report: http://coverage.selectdb-in.cc/coverage/c7c07971af032bf6b91256a1a9af9afd1bad379d_c7c07971af032bf6b91256a1a9af9afd1bad379d/report/index.html