VndErrors.VndError#logRef is not optional

[matlach]

According to the specification https://github.com/blongden/vnd.error

logref

OPTIONAL

For expressing a (numeric/alpha/alphanumeric) identifier to refer to the specific error on the server side for logging purposes (i.e. a request number).

The logref attribute should be optional. Though, in Spring org.springframework.hateoas.VndErrors.VndError:

        /**
         * Creates a new {@link VndError} with the given logref, a message as well as some {@link Link}s.
         * 
         * @param logref must not be {@literal null} or empty.
         * @param message must not be {@literal null} or empty.
         * @param links
         */
        public VndError(String logref, String message, Link... links) {

            Assert.hasText(logref, "Logref must not be null or empty!");
            Assert.hasText(message, "Message must not be null or empty!");

            this.logref = logref;
            this.message = message;
            this.add(Arrays.asList(links));
        }

[gregturn]

Resolved via 4f3be11.