[FIX] account: modal account.invoice.line form traceback

In some instances we could get a traceback in the form view of an
account.invoice.line object. This is because the `parent` field can be
undefined depending of the way the form view was opened.

For example there can be a account.invoice.line list in a
sale.order.line form and this lead to a product_id with a traceback
capable context.

addons/account/views/account_invoice_view.xml

@@ -61,7 +61,7 @@
                     <group>
                         <group>
                             <field name="product_id"
-                                 context="{'partner_id': parent.partner_id}"/>
+                                 context="parent and {'partner_id': parent.partner_id}"/>
                             <label for="quantity"/>
                             <div>
                                 <field name="quantity" class="oe_inline"/>