[FIX] crm: fix bug introduced by new api in #aa44700

Before migration:
  'phone': customer and customer.phone or lead.phone,
After migration:
  'phone': customer.phone if customer else self.phone,

So if a customer exists and that it has no phone, the phone
on the lead need to be preserved during convertion in opp.

addons/crm/models/crm_lead.py

@@ -689,8 +689,8 @@ class Lead(FormatAddress, models.Model):
             'partner_id': customer.id if customer else False,
             'type': 'opportunity',
             'date_open': fields.Datetime.now(),
-            'email_from': customer.email if customer else self.email_from,
-            'phone': customer.phone if customer else self.phone,
+            'email_from': customer and customer.email or self.email_from,
+            'phone': customer and customer.phone or self.phone,
             'date_conversion': fields.Datetime.now(),
         }
         if not self.stage_id: