[dataimport] make it possible to insert 'empty' (i.e. no-attrs) entities In such cases (e.g. ternary relations), generated RQL must be 'INSERT MyEntityType' instead of 'INSERT MyEntityType:'

addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile