fix: Check if items exist (#16248)

diff --git a/erpnext/controllers/buying_controller.py b/erpnext/controllers/buying_controller.py
index 7ed225f..0496ab2 100644
--- a/erpnext/controllers/buying_controller.py
+++ b/erpnext/controllers/buying_controller.py
@@ -462,7 +462,7 @@
 		update_last_purchase_rate(self, is_submit = 0)
 
 	def validate_schedule_date(self):
-		if not self.schedule_date:
+		if not self.schedule_date and self.get("items"):
 			self.schedule_date = min([d.schedule_date for d in self.get("items")])
 
 		if self.schedule_date: