Adding invoice line no picking name 7

moved from https://code.launchpad.net/~agilebg/account-invoicing/adding_invoice_line_no_picking_name_7/+merge/220289
It's usually desirable to initialise context with a dictionary before propagating it:

Like:
```python
context = context or {}
```
or
```python
if context is None:
    context = {}
```