Fix map nullable flag in ParquetTypeConverter

[viirya]

Which issue does this PR close?

Closes #1587.

Rationale for this change

What changes are included in this PR?

Are there any user-facing changes?

[viirya]

Due to previous inconsistency I described in the ticket, I changed this to follow visit_map.

[codecov-commenter]

Codecov Report

Merging #1592 (bc0cb98) into master (db4cb8f) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head bc0cb98 differs from pull request most recent head 380ba0a. Consider uploading reports for the commit 380ba0a to get more accurate results

@@            Coverage Diff             @@
##           master    #1592      +/-   ##
==========================================
+ Coverage   82.93%   82.95%   +0.01%     
==========================================
  Files         193      193              
  Lines       55350    55376      +26     
==========================================
+ Hits        45907    45939      +32     
+ Misses       9443     9437       -6     

Impacted Files	Coverage Δ
arrow/src/compute/kernels/substring.rs	100.00% <100.00%> (+1.68%)	⬆️
parquet/src/arrow/schema.rs	86.08% <100.00%> (+0.24%)	⬆️
arrow/src/array/transform/mod.rs	86.57% <0.00%> (+0.22%)	⬆️
parquet_derive/src/parquet_field.rs	66.43% <0.00%> (+0.22%)	⬆️

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[tustvold]

I've confirmed this aligns with the specification - https://github.com/apache/parquet-format/blob/master/LogicalTypes.md

[viirya]

Thank you @tustvold for confirming it.