fix(gin): Do not panic when handling method not allowed on empty tree

[phsym]

Fix #4002

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.06%. Comparing base (3dc1cd6) to head (a3b386f).
Report is 64 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4003      +/-   ##
==========================================
- Coverage   99.21%   99.06%   -0.16%     
==========================================
  Files          42       44       +2     
  Lines        3182     2772     -410     
==========================================
- Hits         3157     2746     -411     
+ Misses         17       15       -2     
- Partials        8       11       +3     

Flag	Coverage Δ
	?
-tags "sonic avx"	99.05% <100.00%> (?)
-tags go_json	99.05% <100.00%> (?)
-tags nomsgpack	99.04% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	99.06% <100.00%> (-0.16%)	⬇️
go-1.22	99.06% <100.00%> (?)
macos-latest	99.06% <100.00%> (-0.16%)	⬇️
ubuntu-latest	99.06% <100.00%> (-0.16%)	⬇️

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