[dataimport] backout 6947201033be (related to #2788402) (and add a try: except to cache the intended error) The problem actually comes from the ``MassiveObjectStore`` in the ``dataio`` cube, so it should be corrected there. Here, we only protect it with a ``RuntimeWarning`` so that the user can see the problem. ``value`` is set to ``None`` (whence to ``NULL`` from a database standpoint), so that the data can be nevertheless inserted in the database. However, only the keys present in ``row`` are actually non-'``NULL``'. The real solution is to work out the issue in ``MassiveObjectStore`` directly. The current try/except should only be a temporary hack.