[dataimport] _create_copyfrom_buffer: fix separator check in string converter
The empty string is a valid replace_sep.
Related to #3845572
for table, column in [
('transactions', 'tx_time'),
('tx_entity_actions', 'tx_uuid'),
('tx_relation_actions', 'tx_uuid')]:
repo.system_source.create_index(session, table, column)
commit()