[req] if a user has a None property, fall back to the default
This can happen for InternalManager; the normal CWUser implementation of
property_value already has a similar fallback. Related to #3381521
usr/lib/PY_VERSION/*-packages/cubicweb/entities/
usr/lib/PY_VERSION/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/PY_VERSION/*-packages/cubicweb/*.py