Fix: Null buffer accounts for offset in substring kernel.

[HaoYang670]

Which issue does this PR close?

Closes #1639.

Rationale for this change

The null buffer was not aligned with the offset of array.

What changes are included in this PR?

Use bit_slice to align the null buffer.
Add tests to test the alignment.

Are there any user-facing changes?

No.

[codecov-commenter]

Codecov Report

Merging #1704 (2840b5a) into master (ff182f1) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1704      +/-   ##
==========================================
+ Coverage   83.27%   83.29%   +0.01%     
==========================================
  Files         195      195              
  Lines       55937    55981      +44     
==========================================
+ Hits        46584    46629      +45     
+ Misses       9353     9352       -1     

Impacted Files	Coverage Δ
arrow/src/compute/kernels/substring.rs	99.41% <100.00%> (+0.05%)	⬆️
arrow/src/array/transform/mod.rs	86.80% <0.00%> (-0.12%)	⬇️
parquet_derive/src/parquet_field.rs	66.21% <0.00%> (+0.22%)	⬆️
arrow/src/datatypes/datatype.rs	65.45% <0.00%> (+0.36%)	⬆️

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[viirya]

Thanks @HaoYang670

[alamb]

Thank you @HaoYang670

[alamb]

(nice tests)

[HaoYang670]

Thank you for your review @viirya @alamb !!