Fix LP1177916, Cannot activate PO which contains only direct charges

Fix LP1177916, Cannot activate PO which contains only direct charges
We add a safety check in the function
Application/Acq/Order.pm/create_lineitem_list_assets(), which is called
by create_po_assets(), which is the service call initiated by the user
trying to activate a PO.

The safety check prevents the function from processing if there are no
line items specified in the arguments.

P.S. It would be better to stop the sequence of events earlier in the
client, but that will need more coding, which can been done in another
fix.

Signed-off-by: Steven Chan <schan@sitka.bclibraries.ca>