ARROW-11271: [Rust] [Parquet] Fix parquet list schema null conversion

[nevi-me]

Please see the JIRA for the issue details https://issues.apache.org/jira/browse/ARROW-11271.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-11271

[nevi-me]

This is not as severe as i thought, so it's fine if it doesn't make it to 3.0.0 @alamb @kszucs

[codecov-io]

Codecov Report

Merging #9216 (6966a9a) into master (1393188) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #9216   +/-   ##
=======================================
  Coverage   81.61%   81.61%           
=======================================
  Files         215      215           
  Lines       51867    51891   +24     
=======================================
+ Hits        42329    42351   +22     
- Misses       9538     9540    +2     

Impacted Files	Coverage Δ
rust/parquet/src/arrow/schema.rs	91.66% <100.00%> (+0.16%)	⬆️
rust/parquet/src/encodings/encoding.rs	94.86% <0.00%> (-0.20%)	⬇️

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[kszucs]

This is not as severe as i thought, so it's fine if it doesn't make it to 3.0.0 @alamb @kszucs

We have packaging issues so we probably need to create another release candidate. We can include this patch as well.

[alamb]

I reviewed the code in this PR and I think it does what the description says. It also seems to be a relatively small change with minimal impacts (and reduces the number of panic! in the code). I think we can merge it. I will do so