HTMLRender interface: Instance method needs to return an error type

[ljluestc]

This PR updates the HTMLDebug struct in render/html.go to return nil from loadTemplate instead of panicking when no files or glob patterns are provided. This change makes the behavior more robust and eliminates the need for external files in some test cases.

Changes:

• Modified render/html.go:
  • loadTemplate now returns nil instead of panicking when Files and Glob are empty.
  • Render handles a nil Template gracefully by returning nil.
• Updated render/html_test.go:
  • TestHTMLDebugInstanceSuccess and TestHTMLDebugInstanceNoFilesOrGlob expect a nil Template when no files are provided, avoiding file dependencies.
• Updated render/render_test.go:
  • Changed TestRenderHTMLDebugPanics to expect a nil Template instead of a panic, aligning with the new behavior.

Motivation:

• Eliminates test failures due to missing testdata/template.html without requiring new files.
• Improves robustness by avoiding unnecessary panics in debug mode.

Testing:

• Verified locally with go test -v ./render/... in the gin repository.
• All tests pass without external file dependencies.

[codecov]

Codecov Report

Attention: Patch coverage is 35.71429% with 9 lines in your changes missing coverage. Please review.

Project coverage is 98.73%. Comparing base (3dc1cd6) to head (a4006e2).
Report is 142 commits behind head on master.

Files with missing lines	Patch %	Lines
render/html.go	35.71%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4210      +/-   ##
==========================================
- Coverage   99.21%   98.73%   -0.48%     
==========================================
  Files          42       44       +2     
  Lines        3182     3411     +229     
==========================================
+ Hits         3157     3368     +211     
- Misses         17       30      +13     
- Partials        8       13       +5     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags "sonic avx"	98.73% <35.71%> (?)
-tags go_json	98.73% <35.71%> (?)
-tags nomsgpack	98.72% <35.71%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.23	98.73% <35.71%> (?)
go-1.24	98.73% <35.71%> (?)
macos-latest	98.73% <35.71%> (-0.48%)	⬇️
ubuntu-latest	98.73% <35.71%> (-0.48%)	⬇️

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