[FIX](decimal)fix column is decimalv3 cast to decimalv2 make be core

[amorynan]

Proposed changes

Issue Number: close #xxx
when after cast expr , block column is nullable, so decimalv2 serde should handle nullable

Further comments

If this is a relatively large or complex change, kick off the discussion at dev@doris.apache.org by explaining why you chose the solution you did and what alternatives you considered, etc...

[amorynan]

run buildall

[github-actions]

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

[jacktengg]

LGTM

[github-actions]

PR approved by anyone and no changes requested.

[doris-robot]

TeamCity be ut coverage result:
Function Coverage: 37.34% (8468/22681)
Line Coverage: 29.65% (68543/231157)
Region Coverage: 28.23% (35598/126111)
Branch Coverage: 25.10% (18224/72618)
Coverage Report: http://coverage.selectdb-in.cc/coverage/54ae62db70cd59a5d16dc8fa54883d705eb483cd_54ae62db70cd59a5d16dc8fa54883d705eb483cd/report/index.html

[doris-robot]

(From new machine)TeamCity pipeline, clickbench performance test result:
the sum of best hot time: 46.22 seconds
stream load tsv: 553 seconds loaded 74807831229 Bytes, about 129 MB/s
stream load json: 20 seconds loaded 2358488459 Bytes, about 112 MB/s
stream load orc: 64 seconds loaded 1101869774 Bytes, about 16 MB/s
stream load parquet: 33 seconds loaded 861443392 Bytes, about 24 MB/s
insert into select: 28.6 seconds inserted 10000000 Rows, about 349K ops/s
storage size: 17162293497 Bytes

[amorynan]

run external

[yiguolei]

LGTM

[jacktengg]

LGTM

[github-actions]

PR approved by at least one committer and no changes requested.