Support dynamic schema without __name__

This supports use of getPrefix() result when a schema.__name__ is unavailable (empty, unnamed).  Includes change note and doctest.

Requirement: I have a use-case where a subclass of AutoExtensibleForm for a form application where schema are loaded dynamically, and technical constraints (content-addressed storage of schemas in ZODB keyed on plone.supermodel serialization md5 checksum) prevents the ability to have named schema (when the identifier is based on the checksum of the content, you cannot put the name in the content and still have idempotent result).  Admittedly, this is a non-Dexterity edge-case, but it is tested and important for me to support, and there are no workarounds other than maintaining a fork or monkey patch of plone.autoform, which I would rather not do.