[dataimport] fix db-namespace default value handling In a fresh new instance, the "db-namespace" option is set as empty by default in the "sources" file. This seems correct but empty string is not a valid pg_schema and actually means that we should use "public"

Latest changes 
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* table of `all latest changes`_

Links below is providing useful RQL query example.

.. _all latest changes: ../view?rql=Any+M%2CX+WHERE+X+modification_date+M+ORDERBY+M+DESC+LIMIT+30