Enable nilnil linter for new code (#36591)

Fixes: https://github.com/go-gitea/gitea/issues/36152

Enable the `nilnil` linter while adding `//nolint` comments to existing
violations. This will ensure no new issues enter the code base while we
can fix existing issues gradually.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

models/unittest/fixtures_loader.go

@@ -169,7 +169,7 @@ func (f *fixturesLoaderInternal) Load() error {
 
 func FixturesFileFullPaths(dir string, files []string) (map[string]*FixtureItem, error) {
 	if files != nil && len(files) == 0 {
-		return nil, nil // load nothing
+		return nil, nil //nolint:nilnil // load nothing
 	}
 	files = slices.Clone(files)
 	if len(files) == 0 {

models/unittest/fixtures_test.go

@@ -16,7 +16,7 @@ import (
 )
 
 var NewFixturesLoaderVendor = func(e *xorm.Engine, opts unittest.FixturesOptions) (unittest.FixturesLoader, error) {
-	return nil, nil
+	return nil, nil //nolint:nilnil // no vendor fixtures loader configured
 }
 
 /*