[bugfix] correctly differentiate between nil / present-but-empty slices

[NyaaaWhatsUpDoc]

This fixes the regression introduced in v1.11.0 which stopped differentiating between nil and zero length (but not nil) slices when binding form values.

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[NyaaaWhatsUpDoc]

this isn't part of the fix (nor is it in the array part below), but it just means vs only needs to get written to once instead of twice in the case of cfTag == "" | cfTag == "multi"

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.99%. Comparing base (3dc1cd6) to head (f2443d1).
⚠️ Report is 230 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4498      +/-   ##
==========================================
- Coverage   99.21%   98.99%   -0.22%     
==========================================
  Files          42       44       +2     
  Lines        3182     2992     -190     
==========================================
- Hits         3157     2962     -195     
- Misses         17       19       +2     
- Partials        8       11       +3     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.92% <100.00%> (?)
-tags go_json	98.85% <100.00%> (?)
-tags nomsgpack	98.98% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.24	98.93% <100.00%> (?)
go-1.25	98.99% <100.00%> (?)
macos-latest	98.99% <100.00%> (-0.22%)	⬇️
ubuntu-latest	98.93% <100.00%> (-0.29%)	⬇️

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