fix: reset the df schema after read to fix the "not null" issue when loading data

[vagetablechicken]

No description provided.

[tobegit3hub]

LGTM

[github-actions]

Linux Test Report

     102 files       228 suites   43m 0s ⏱️
  8 593 tests   8 590 ✔️ 3 💤 0 ❌
12 715 runs  12 712 ✔️ 3 💤 0 ❌

Results for commit 9f88fb7.

[codecov]

Codecov Report

Merging #1341 (9f88fb7) into branch-0.4 (d4a07c5) will decrease coverage by 0.05%.
The diff coverage is 77.61%.

@@               Coverage Diff                @@
##             branch-0.4    #1341      +/-   ##
================================================
- Coverage         65.50%   65.45%   -0.06%     
  Complexity          222      222              
================================================
  Files               570      570              
  Lines            106347   106386      +39     
  Branches            841      854      +13     
================================================
- Hits              69664    69636      -28     
- Misses            36537    36604      +67     
  Partials            146      146              

Impacted Files	Coverage Δ
...openmldb/taskmanager/config/TaskManagerConfig.java	0.00% <0.00%> (ø)
...m/_4paradigm/openmldb/taskmanager/dao/JobInfo.java	0.00% <0.00%> (ø)
...nmldb/taskmanager/server/impl/TaskManagerImpl.java	0.00% <0.00%> (ø)
...paradigm/openmldb/taskmanager/JobInfoManager.scala	0.00% <0.00%> (ø)
src/cmd/sql_cmd.h	21.41% <ø> (-0.10%)	⬇️
..._4paradigm/openmldb/batch/utils/HybridseUtil.scala	60.90% <38.46%> (-2.44%)	⬇️
...4paradigm/openmldb/batch/nodes/WindowAggPlan.scala	73.89% <66.66%> (-0.24%)	⬇️
...4paradigm/openmldb/batch/api/OpenmldbSession.scala	57.57% <94.11%> (+4.70%)	⬆️
..._4paradigm/openmldb/batch/nodes/LoadDataPlan.scala	60.37% <100.00%> (+4.58%)	⬆️
..._4paradigm/openmldb/batch/utils/SparkRowUtil.scala	66.66% <100.00%> (+14.03%)	⬆️
... and 14 more

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