Fixed error in reading nullable from Avro.

[jorgecarleitao]

The deserializer assumed that the null variant was always the first of the union, which is not true.

[codecov]

Codecov Report

Merging #631 (070e5e7) into main (439dacf) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #631      +/-   ##
==========================================
- Coverage   69.78%   69.77%   -0.02%     
==========================================
  Files         299      299              
  Lines       16667    16704      +37     
==========================================
+ Hits        11631    11655      +24     
- Misses       5036     5049      +13     

Impacted Files	Coverage Δ
src/io/avro/read/deserialize.rs	72.64% <100.00%> (+1.47%)	⬆️
src/array/fixed_size_binary/mod.rs	73.00% <0.00%> (-6.72%)	⬇️
src/bitmap/utils/slice_iterator.rs	91.04% <0.00%> (-1.50%)	⬇️
src/compute/arithmetics/time.rs	46.93% <0.00%> (+2.04%)	⬆️

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