Omit validity buffer in PrimitiveArray::from_iter when all values are valid

[jhorstmann]

Which issue does this PR close?

Closes #1856.

Rationale for this change

This shouldn't add any overhead, but reduces memory usage slightly and maybe makes iteration a bit faster by not having to check a null buffer

What changes are included in this PR?

Are there any user-facing changes?

The behavior change can be observed by checking the memory consumption, like in the test_memory_size_primitive_nullable test that had to be adjusted, but semantically there is no change.

[codecov-commenter]

Codecov Report

Merging #1859 (7bfc3f6) into master (fb697ce) will increase coverage by 0.00%.
The diff coverage is 95.00%.

@@           Coverage Diff           @@
##           master    #1859   +/-   ##
=======================================
  Coverage   83.49%   83.49%           
=======================================
  Files         201      201           
  Lines       56902    56915   +13     
=======================================
+ Hits        47511    47524   +13     
  Misses       9391     9391           

Impacted Files	Coverage Δ
arrow/src/array/array_primitive.rs	94.97% <94.44%> (-0.09%)	⬇️
arrow/src/array/array.rs	89.69% <100.00%> (+0.02%)	⬆️
parquet_derive/src/parquet_field.rs	65.75% <0.00%> (-0.23%)	⬇️
arrow/src/array/array_struct.rs	89.20% <0.00%> (+0.79%)	⬆️

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