[FIX] mrp: successfully retrieve empty computed field

1. Install MRP
2. Launch an Odoo shell on the database
3. Type the command
`print(env["mrp.bom.line"].browse(1).child_line_ids)`

Traceback will occur with exception
odoo.exceptions.CacheMiss: ('mrp.bom.line(1,).child_line_ids', None)

This occur due to the empty value that the computation of the field
yields, which is not saved in cache.
Fix by adding a default value.

opw-2226951

closes odoo/odoo#50950

X-original-commit: fc885499a3c75d8d564569ec4cea41a8a64f3164
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/mrp/models/mrp_bom.py

@@ -325,7 +325,7 @@ class MrpBomLine(models.Model):
     def _compute_child_line_ids(self):
         """ If the BOM line refers to a BOM, return the ids of the child BOM lines """
         for line in self:
-            line.child_line_ids = line.child_bom_id.bom_line_ids.ids
+            line.child_line_ids = line.child_bom_id.bom_line_ids.ids or False
 
     @api.onchange('product_uom_id')
     def onchange_product_uom_id(self):