Further improve the nil value handling in arrays

[gwynne]

These changes are now available in 2.16.1

The previous set of fixes was incomplete. This revision correctly handles types which do not accidentally end up going through the legacy fast path via conforming to the deprecated PostgresDataConvertible, as many built-in types still do. The slow path through Encodable can now detect the correct psqlArrayType for all-nil arrays as long as the underlying Element type conforms to PostgresArrayEncodable.

[codecov]

Codecov Report

❌ Patch coverage is 87.71930% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.90%. Comparing base (a8ce958) to head (3527c2d).

Files with missing lines	Patch %	Lines
Sources/PostgresKit/PostgresDataTranslation.swift	87.71%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #280      +/-   ##
==========================================
+ Coverage   43.37%   45.90%   +2.52%     
==========================================
  Files          13       13              
  Lines         853      891      +38     
==========================================
+ Hits          370      409      +39     
+ Misses        483      482       -1     

Files with missing lines	Coverage Δ
Sources/PostgresKit/PostgresDataTranslation.swift	73.70% <87.71%> (+3.33%)	⬆️

... and 1 file with indirect coverage changes

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