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>

routers/api/packages/swift/swift.go

@@ -300,7 +300,7 @@ func formFileOptionalReadCloser(ctx *context.Context, formKey string) (io.ReadCl
 
 	content := ctx.Req.FormValue(formKey)
 	if content == "" {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil to indicate that the content does not exist
 	}
 	return io.NopCloser(strings.NewReader(content)), nil
 }