fix: avoid an allocation in GetQuery* when query string is empty

[ItalyPaleAle]

When requesting a URL without a query string (e.g. http://example.com/), Gin was allocating a map in the heap unnecessarily, which was showing up on our benchmarks

Pull Request Checklist

Please ensure your pull request meets the following requirements:

• Open your pull request against the master branch.
• All tests pass in available continuous integration systems (e.g., GitHub Actions).
• Tests are added or modified as needed to cover code changes.
• (N/A) If the pull request introduces a new feature, the feature is documented in the docs/doc.md.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.33%. Comparing base (3dc1cd6) to head (b8e1de3).
⚠️ Report is 289 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4765      +/-   ##
==========================================
- Coverage   99.21%   98.33%   -0.89%     
==========================================
  Files          42       48       +6     
  Lines        3182     3176       -6     
==========================================
- Hits         3157     3123      -34     
- Misses         17       43      +26     
- Partials        8       10       +2     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.31% <100.00%> (?)
-tags go_json	98.25% <100.00%> (?)
-tags nomsgpack	98.30% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.33% <100.00%> (?)
go-1.26	98.33% <100.00%> (?)
macos-latest	98.33% <100.00%> (-0.89%)	⬇️
ubuntu-latest	98.33% <100.00%> (-0.89%)	⬇️

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