[fix](FixedString) fix type char error when be config::enable_low_cardinality_optimize = false

[Mryange]

Proposed changes

Issue Number: close #xxx

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...

[Mryange]

run buildall

[github-actions]

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

[doris-robot]

TeamCity be ut coverage result:
Function Coverage: 37.84% (7993/21123)
Line Coverage: 29.52% (64857/219671)
Region Coverage: 28.99% (33367/115092)
Branch Coverage: 24.89% (17129/68830)
Coverage Report: http://coverage.selectdb-in.cc/coverage/fce93f22507a474996dcde9b660eb11fbb6fc869_fce93f22507a474996dcde9b660eb11fbb6fc869/report/index.html

[Mryange]

run buildall

[doris-robot]

TeamCity be ut coverage result:
Function Coverage: 37.82% (7990/21124)
Line Coverage: 29.50% (64830/219735)
Region Coverage: 28.97% (33353/115121)
Branch Coverage: 24.87% (17124/68850)
Coverage Report: http://coverage.selectdb-in.cc/coverage/fce93f22507a474996dcde9b660eb11fbb6fc869_fce93f22507a474996dcde9b660eb11fbb6fc869/report/index.html

[github-actions]

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

[github-actions]

PR approved by anyone and no changes requested.