[13.0][FIX] purchase_warn_message: don't show empty warning

Author: HviorForgeFlow

purchase_warn_message/models/purchase_order.py

@@ -29,4 +29,4 @@ def _compute_purchase_warn_msg(self):
                     purchase_warn_msg += p.purchase_warn_msg
                 if p != rec.partner_id and rec.partner_id.purchase_warn == "warning":
                     purchase_warn_msg += separator + rec.partner_id.purchase_warn_msg
-            rec.purchase_warn_msg = purchase_warn_msg
+            rec.purchase_warn_msg = purchase_warn_msg or False