Acq: Avoid failure in processing EDI invoices when vendor sends non-numeric PO identifier

[Lebbeous Fogle-Weekley]

For Evergreen 2.3+:

http://git.evergreen-ils.org/?p=working/Evergreen.git;a=shortlog;h=refs/heads/collab/senator/edi-invoice-bad-po-number

This is a fix to the fix for bug 1172373, specifically case "2" from the report's main "bug description" body.

The earlier fix avoided an early error when the system would attempt to save a string like "HODOR", or whatever the vendor puts into the invoice, in an INT column in the database, but later in our code path the string would get cast to an int (0 of course). Putting that into the same column also fails because the column is a foreign key reference to a table where no rows have ID 0.

Anyway, this patch allows start-to-finish success in processing EDI invoices where the vendor has inappropriately put something that isn't a PO number into the RFF+ON segment

[Ben Shum (bshum)]

Picked to master, rel_2_4, and rel_2_3.