Fix rel csr sliding out-of-place commit and null strings

[ray6080]

1. Inside distributeAndUpdateColumn, set new chunk to all null before copy existing values and inserting new ones.
2. Fix Column::commitColumnChunkOutOfPlace.
3. Skip null strings in StringColumn::canCommitInPlace.

[codecov]

Codecov Report

Attention: Patch coverage is 89.47368% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 93.27%. Comparing base (ff186a5) to head (0b7adb9).

Files	Patch %	Lines
src/storage/store/column_chunk.cpp	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3055      +/-   ##
==========================================
+ Coverage   93.25%   93.27%   +0.01%     
==========================================
  Files        1133     1133              
  Lines       43117    43123       +6     
==========================================
+ Hits        40209    40221      +12     
+ Misses       2908     2902       -6     

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