Remove null_count from write_batch_with_statistics

[tustvold]

Which issue does this PR close?

Closes #2046

Rationale for this change

We must compute null counts regardless, and so there is no advantage to providing a pre-computed null count

What changes are included in this PR?

Removes null count from write_batch_with_statistics

Are there any user-facing changes?

Yes

[codecov-commenter]

Codecov Report

Merging #2047 (b635f0d) into master (ca5fe7d) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2047      +/-   ##
==========================================
+ Coverage   83.54%   83.56%   +0.01%     
==========================================
  Files         222      222              
  Lines       58186    58211      +25     
==========================================
+ Hits        48612    48642      +30     
+ Misses       9574     9569       -5     

Impacted Files	Coverage Δ
parquet/src/column/writer.rs	91.53% <100.00%> (+0.05%)	⬆️
arrow/src/array/equal/mod.rs	96.48% <0.00%> (+0.28%)	⬆️
arrow/src/ffi.rs	87.47% <0.00%> (+0.30%)	⬆️
arrow/src/datatypes/datatype.rs	65.68% <0.00%> (+0.36%)	⬆️
arrow/src/datatypes/ffi.rs	75.00% <0.00%> (+2.27%)	⬆️

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[ursabot]

Benchmark runs are scheduled for baseline = a2f223c and contender = 88e0de5. 88e0de5 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java