[FIX] mrp: compute duration when needed

Before this commit there would be trackback (TypeError: unsupported operand type(s) for -: 'datetime.datetime' and 'NoneType') when you will try to remove field Start date as we are using the field to compute duration.

With this commit, we compute duration when Both fields are present.

closes odoo/odoo#59693

Signed-off-by: Rémy Voet <ryv-odoo@users.noreply.github.com>

addons/mrp/models/mrp_workcenter.py

@@ -270,7 +270,7 @@ class MrpWorkcenterProductivity(models.Model):
     @api.depends('date_end', 'date_start')
     def _compute_duration(self):
         for blocktime in self:
-            if blocktime.date_end:
+            if blocktime.date_start and blocktime.date_end:
                 d1 = fields.Datetime.from_string(blocktime.date_start)
                 d2 = fields.Datetime.from_string(blocktime.date_end)
                 diff = d2 - d1