[web/json] an empty rset is just fine for ejsonexport
The ejsonexport view can just return an empty list for an empty rset.
Closes #4005518
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/