Fix comparisons between non-nil values and nil

[gwynne]

These changes are now available in 1.13.1

The changes in #135 introduced a nasty bug where conditions of the form #if (value == nil) or #if (value != nil) would incorrectly treat nil as equal to any non-nil value. Unfortunately, there was no test for this case, so the problem was not caught. This fixes the issue and adds the missing test.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.78%. Comparing base (73f4ad4) to head (0df0f64).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #138      +/-   ##
==========================================
- Coverage   80.84%   80.78%   -0.06%     
==========================================
  Files          25       25              
  Lines        2678     2665      -13     
==========================================
- Hits         2165     2153      -12     
+ Misses        513      512       -1     

Files with missing lines	Coverage Δ
Sources/LeafKit/LeafData/LeafData.swift	62.23% <100.00%> (+0.30%)	⬆️

... and 2 files with indirect coverage changes

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