[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"
\.svn
^build$
^dist$
\.egg-info$
^.tox$
\.pyc$
\.pyo$
\.bak$
\.old$
\~$
\#.*?\#$
\.swp$
^doc/book/en/apidoc$
\.old$
syntax: regexp
.*/data.*/database/.*\.sqlite
.*/data.*/database/.*\.config
.*/data/database/tmpdb.*
.*/data/ldapdb/.*
.*/data/uicache/
.*/data/cubes/.*/i18n/.*\.po
^doc/html/
^doc/doctrees/
^doc/book/en/devweb/js_api/
^doc/_build
^doc/js_api/
data/pgdb/
data.*/pgdb.*