transmogrify cm_org_identities affiliation may be NULL or empty string

In Registry 4.x the OrgIdentity affiliation is not required and allowed to be empty. As a consequence the affiliation column in the table cm_org_identities may have rows with values that are NULL or the empty string.

Transmogrify is using the same logic to map OrgIdentity (ExternalIdentity) affiliation as CoPersonRole affiliation and is not able to properly process the empty strings or NULL values.