[FIX] stock: prevent traceback when lots is empty in import serial/lots

When the user click on the 'import' button without lots in the import
serial/lots wizard in stock then this issue is generated.

Steps to reproduce:

- Install Inventory.
- Inventory >> Operations >> Receipts >> create the new Receipt
- Add product to the receipt which has tracking 'by lots',
  Save the Receipts
- Click on the button in a tree view which opens the detailed operation wizard,
  And click on Import Serial/Lots in it
- In Import Serial/ Lots wizard click on the import button without passing any
  value in the lots field
- Traceback will be generated

Applying these changes will resolve this issue.

sentry-4244975188

closes odoo/odoo#125786

X-original-commit: a45aeea3335a96390de0b4b5760683e3081852df
Signed-off-by: William Henrotin (whe) <whe@odoo.com>

addons/stock/wizard/stock_import_lot_views.xml

@@ -16,7 +16,7 @@
                     <field name="picking_code" invisible="1"/>
                     <field name="move_id" invisible="1"/>
                     <field name="move_location_dest_id" invisible="1"/>
-                    <field name="lots"/>
+                    <field name="lots" required="1"/>
                     <field name="location_dest_id" attrs="{'invisible': [('picking_code', '=', 'outgoing')]}"/>
                 </group>
             </group>