Correct nullable in read_dictionary

[viirya]

Which issue does this PR close?

Closes #1892.

Rationale for this change

What changes are included in this PR?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #1893 (5a7ffee) into master (2e3d8dd) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 5a7ffee differs from pull request most recent head 7f9d407. Consider uploading reports for the commit 7f9d407 to get more accurate results

@@            Coverage Diff             @@
##           master    #1893      +/-   ##
==========================================
- Coverage   83.48%   83.48%   -0.01%     
==========================================
  Files         203      203              
  Lines       57141    57141              
==========================================
- Hits        47705    47703       -2     
- Misses       9436     9438       +2     

Impacted Files	Coverage Δ
arrow/src/ipc/reader.rs	90.82% <100.00%> (ø)
arrow/src/datatypes/datatype.rs	65.42% <0.00%> (-0.38%)	⬇️
parquet_derive/src/parquet_field.rs	65.53% <0.00%> (-0.23%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2e3d8dd...7f9d407. Read the comment docs.