Calculate nbytes for empty array correctly

[andyferris]

Previously it was assuming the byte header and and terminating 0 would be written for an empty array (for a total of 3 bytes instead of 1).

Fixes #9

[codecov]

Codecov Report

Merging #10 (c42b4bc) into main (a95e23f) will increase coverage by 0.31%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #10      +/-   ##
==========================================
+ Coverage   77.52%   77.83%   +0.31%     
==========================================
  Files          13       13              
  Lines         783      794      +11     
==========================================
+ Hits          607      618      +11     
  Misses        176      176              

Impacted Files	Coverage Δ
src/types/arrays.jl	65.88% <100.00%> (+0.82%)	⬆️
src/types/records.jl	87.50% <0.00%> (+0.32%)	⬆️
src/types/binary.jl	85.71% <0.00%> (+0.32%)	⬆️
src/types/fixed.jl	84.00% <0.00%> (+0.66%)	⬆️
src/utils.jl	82.75% <0.00%> (+1.98%)	⬆️

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[quinnj]

Thanks!