Correcting the set of numValues for column chunk in CopyRel and fix writing null struct entry

[ray6080]

1. numValues of the csrOffsetColumnChunk / adjColumnChunk should be set according to numNodes from source node table that falls into this node group.
2. when a struct entry is null, writing it would trigger unnecessary out-of-place commits for children fields, due to that the input Vector doesn't guarantee its child vector are all set to null. fix this by skipping writing values of children fields, instead setting them all to null.

[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (7ef3d5c) 90.86% compared to head (64a6c4b) 90.85%.
Report is 2 commits behind head on master.

Files	Patch %	Lines
src/storage/store/struct_column.cpp	0.00%	8 Missing ⚠️
src/storage/store/node_group.cpp	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2445      +/-   ##
==========================================
- Coverage   90.86%   90.85%   -0.01%     
==========================================
  Files        1023     1023              
  Lines       37044    37063      +19     
==========================================
+ Hits        33659    33674      +15     
- Misses       3385     3389       +4     

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