[FIX] rma_sale: fix CacheError in _prepare_rma_values()

Lesser fix for sale.order.line.rma.wizard methods.
When method '_prepare_rma_values' was called upon records where field 'picking_id' was empty, Odoo raised a CacheError when trying to access field 'move_id'.
That happened because computed method '_compute_move_id' was not assigning a proper value to such field when 'picking_id' was empty.
Once the computed method is fixed (by simply assigning 'False' as 'move_id' value when no picking is set), the CacheError is solved.

rma_sale/wizard/sale_order_rma_wizard.py

@@ -130,6 +130,8 @@ class SaleOrderLineRmaWizard(models.TransientModel):
                         and r.sale_line_id.order_id == record.order_id
                     )
                 )
+            else:
+                record.move_id = False
 
     @api.depends("order_id")
     def _compute_allowed_product_ids(self):