Fix an issue that led to a crash, if an empty array was decoded

[fabianfett]

Thanks @calebkleveter for reporting this

[codecov]

Codecov Report

Merging #52 into master will increase coverage by 0.05%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   90.68%   90.73%   +0.05%     
==========================================
  Files          40       40              
  Lines        3349     3378      +29     
==========================================
+ Hits         3037     3065      +28     
- Misses        312      313       +1     

Impacted Files	Coverage Δ
...PureSwiftJSONTests/Decoding/JSONDecoderTests.swift	93.25% <94.44%> (+0.30%)	⬆️
...ftJSON/Decoding/JSONUnkeyedDecodingContainer.swift	95.23% <100.00%> (+0.04%)	⬆️
...PureSwiftJSONTests/Encoding/JSONEncoderTests.swift	94.25% <100.00%> (+0.50%)	⬆️
...ux/debug/testlist.derived/PureSwiftJSONTests.swift	100.00% <0.00%> (ø)
...kageTestsTestlist.derived/PureSwiftJSONTests.swift	100.00% <0.00%> (ø)

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