Allowed passing None as ipc_fields in flight API

[jorgecarleitao]

This was a regression vs 0.8 - users should be allowed to pass Option to retain the previous behavior.

[codecov]

Codecov Report

Merging #780 (1397210) into main (9f25174) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #780      +/-   ##
==========================================
- Coverage   71.07%   71.06%   -0.01%     
==========================================
  Files         316      316              
  Lines       16755    16760       +5     
==========================================
+ Hits        11908    11911       +3     
- Misses       4847     4849       +2     

Impacted Files	Coverage Δ
src/io/flight/mod.rs	0.00% <0.00%> (ø)
src/compute/arithmetics/time.rs	26.60% <0.00%> (+0.91%)	⬆️
src/io/parquet/read/nested_utils.rs	78.43% <0.00%> (+0.98%)	⬆️
src/io/avro/read/schema.rs	55.91% <0.00%> (+1.07%)	⬆️

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