boolean isNullClaim(String name) {
    JwtNames.validate(name);
    try {
      return JsonNull.INSTANCE.equals(payload.get(name));
    } catch (JsonParseException ex) {
      return false;
    }
  }
Adds a custom claim encoded in a JSON {@code String} to the JWT.
java_src/src/main/java/com/google/crypto/tink/jwt/RawJwt.java