Fixed error in displaying dictionaries with nulls in values

[jorgecarleitao]

They were displayed as their values, even if they were null.

[codecov]

Codecov Report

Merging #334 (6ebce50) into main (a9932a6) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #334      +/-   ##
==========================================
+ Coverage   80.95%   80.96%   +0.01%     
==========================================
  Files         325      325              
  Lines       21069    21081      +12     
==========================================
+ Hits        17057    17069      +12     
  Misses       4012     4012              

Impacted Files	Coverage Δ
src/array/display.rs	57.89% <ø> (ø)
tests/it/io/print.rs	100.00% <100.00%> (ø)

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