lexgen: encode empty union as util.LexiconTypeDecoder

lex/type_schema.go

···
 			return "*util.LexiconTypeDecoder", nil
 		}
 	case "union":
-		return "*" + name + "_" + strings.Title(k), nil
+		if len(v.Refs) > 0 {
+			return "*" + name + "_" + strings.Title(k), nil
+		} else {
+			// an empty union is effectively an 'unknown', but with mandatory type indicator
+			return "*util.LexiconTypeDecoder", nil
+		}
 	case "blob":
 		return "*util.LexBlob", nil
 	case "array":