[entity] do not raise an AssertionError if the kwargs given to set_relations/attributes is empty
Regression was introduced in 3.16 with the new cw_set API.
Old set_relations and set_attributes methods did not crash on empty
kwargs.
Closes #3104019.
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/