Preserve supplier name when using import line items on draft POs

When creating a draft PO, we first select a supplier to generate the PO number, then use the import line items feature to scan the invoice onto that draft.

Currently, the import process clears out the previously selected supplier name, requiring us to re-enter it. Since supplier name is a required field to save a draft, this creates unnecessary extra work.

Requested Change:
Keep the originally selected supplier name in place when using the import line items feature on a draft PO.