Treat empty timezone string as no-timezone

[dbr]

As per #903 (comment)

[jorgecarleitao]

Thank you! The code is much cleaner, also!

[codecov]

Codecov Report

Merging #1009 (9ec6dbb) into main (5725fa3) will increase coverage by 0.00%.
The diff coverage is 27.45%.

@@           Coverage Diff           @@
##             main    #1009   +/-   ##
=======================================
  Coverage   71.45%   71.45%           
=======================================
  Files         356      356           
  Lines       19758    19762    +4     
=======================================
+ Hits        14118    14121    +3     
- Misses       5640     5641    +1     

Impacted Files	Coverage Δ
src/array/primitive/fmt.rs	68.65% <ø> (ø)
src/ffi/schema.rs	67.62% <27.45%> (+0.09%)	⬆️
src/bitmap/utils/slice_iterator.rs	86.20% <0.00%> (-1.73%)	⬇️
src/compute/arithmetics/time.rs	26.60% <0.00%> (+0.91%)	⬆️

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