Fix AuthorName.setMiddleName to tolerate null middle names

[paulalbert1]

Problem

AuthorName.setMiddleName throws IllegalArgumentException on null. The AWS SDK v1 DynamoDBMapper calls this setter during unconvert, so any Identity whose stored AuthorName has a null middle name fails to deserialize:

DynamoDBMappingException: Identity[identity]; could not unconvert attribute
  caused by: java.lang.IllegalArgumentException: middle name should not be null.
    at reciter.model.identity.AuthorName.setMiddleName
    at ...DynamoDbCrudRepository.findById
    at reciter.service.dynamo.IdentityServiceImpl.findByUid
    at reciter.controller.ExternalArticleController.addExternalArticle

This surfaces as an HTTP 500 from POST /reciter/external-article/by/uid (and GET /reciter/find/identity/by/uid) for every person with a null middle name. In a sampled cohort of 162 people, 123 (76%) hit it — so the external-article manual-add feature is broadly unusable, not an edge case.

Fix

Middle name is optional, unlike first/last name (required identity fields), so a deserialization setter must tolerate null. setMiddleName(null) now sets middleName=null, middleInitial="" and returns; non-null behavior is unchanged. setFirstName/setLastName keep their required-field guards.

Follow-up (not in this PR)

• Release a new reciter-identity-model artifact (2.0.11 if unreleased, else 2.0.12).
• Bump ReCiter/pom.xml from 2.0.10 to the released version and redeploy.

mvn compile verified clean.