fix: fix string columns as null for UnsafeRowOpt

[tobegit3hub]

• Resolve Can not get null data for string columns if enable UnsafeRowOpt #1315 .
• Add unit test for null string columns with unsaferow opt.

[github-actions]

HybridSE Linux Test Report

       65 files       231 suites   5m 32s ⏱️
19 138 tests 19 138 ✔️ 0 💤 0 ❌

Results for commit 7fc2e5a.

[github-actions]

HybridSE Mac Test Report

       65 files       231 suites   7m 47s ⏱️
19 138 tests 19 138 ✔️ 0 💤 0 ❌

Results for commit 7fc2e5a.

[github-actions]

Linux Test Report

     106 files       232 suites   43m 18s ⏱️
  8 615 tests   8 612 ✔️ 3 💤 0 ❌
12 742 runs  12 739 ✔️ 3 💤 0 ❌

Results for commit 7fc2e5a.

[codecov]

Codecov Report

Merging #1326 (7fc2e5a) into main (1aecdbc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main    #1326   +/-   ##
=========================================
  Coverage     65.53%   65.54%           
  Complexity      222      222           
=========================================
  Files           570      570           
  Lines        106494   106494           
  Branches        857      857           
=========================================
+ Hits          69790    69797    +7     
+ Misses        36558    36551    -7     
  Partials        146      146           

Impacted Files	Coverage Δ
hybridse/src/codec/type_codec.cc	75.23% <100.00%> (ø)
src/client/tablet_client.cc	35.25% <0.00%> (-0.22%)	⬇️
src/client/ns_client.cc	41.00% <0.00%> (-0.16%)	⬇️
src/nameserver/name_server_impl.cc	37.13% <0.00%> (-0.06%)	⬇️
src/tablet/tablet_impl.cc	37.98% <0.00%> (+0.22%)	⬆️
src/zk/zk_client.cc	82.92% <0.00%> (+0.27%)	⬆️
src/sdk/db_sdk.cc	60.60% <0.00%> (+0.60%)	⬆️
src/rpc/rpc_client.h	66.21% <0.00%> (+1.35%)	⬆️
..._4paradigm/openmldb/batch/utils/HybridseUtil.scala	62.40% <0.00%> (+1.50%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1aecdbc...7fc2e5a. Read the comment docs.