Fix complex type generation when only one sub field is defined

When adding only one sub field in a complex field, the following error occurs at Nuxeo startup:

Caused by: java.lang.ClassCastException: org.nuxeo.ecm.core.schema.types.ListTypeImpl cannot be cast to org.nuxeo.ecm.core.schema.types.ComplexType

This is due to bad generation of sub field elements.