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/db/collation.go

@@ -98,7 +98,7 @@ func CheckCollations(x *xorm.Engine) (*CheckCollationsResult, error) {
 			return nil, err
 		}
 	} else {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil for unsupported database types
 	}
 
 	if res.DatabaseCollation == "" {