add vendoring

vendor/maunium.net/go/mautrix/util/marshal.go

@@ -0,0 +1,30 @@
+package util
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"github.com/tidwall/gjson"
+	"github.com/tidwall/sjson"
+)
+
+// MarshalAndDeleteEmpty marshals a JSON object, then uses gjson to delete empty objects at the given gjson paths.
+//
+// This can be used as a convenient way to create a marshaler that omits empty non-pointer structs.
+// See mautrix.RespSync for example.
+func MarshalAndDeleteEmpty(marshalable interface{}, paths []string) ([]byte, error) {
+	data, err := json.Marshal(marshalable)
+	if err != nil {
+		return nil, err
+	}
+	for _, path := range paths {
+		res := gjson.GetBytes(data, path)
+		if res.IsObject() && len(res.Raw) == 2 {
+			data, err = sjson.DeleteBytes(data, path)
+			if err != nil {
+				return nil, fmt.Errorf("failed to delete empty %s: %w", path, err)
+			}
+		}
+	}
+	return data, nil
+}