public static UserIdentifier of(@NonNull UUID uniqueId, @Nullable String username) {
        Objects.requireNonNull(uniqueId, "uuid");
        if (username == null || username.equalsIgnoreCase("null") || username.isEmpty()) {
            username = null;
        }

        return new UserIdentifier(uniqueId, username);
    }
Creates a {@link UserIdentifier}.

@param uniqueId the uuid of the user
@param username the username of the user, nullable
@return a new identifier
common/src/main/java/me/lucko/luckperms/common/model/UserIdentifier.java