Line totals migration

[mark-prins]

Fixes #4273

👩🏻‍💻 What does this PR do?

The item_code was coming through, not as None but as an empty string.
I've handled the None and is_empty() as equivalent, and requiring calculation. Would be nice to have some sort of serde deserialize empty_as_none to handle this. surprised it hasn't come up before?

The result is this: the translation handles the new invoice as a new invoice and the pricing works:

💌 Any notes for the reviewer?

🧪 Testing

• as per the issue - migrate the test3 site and check outbound shipment Use url Search query params to determine sort, pagination and filter params on Tables #2

📃 Documentation

• Part of an epic: documentation will be completed for the feature as a whole
• No documentation required: no user facing changes or a bug fix which isn't a change in behaviour
• These areas should be updated or checked:
  1.
  2.

[jmbrunskill]

Works like a charm!