fix: typeerror in get_batches_by_oldest for mixed batch expiry

sort on (expiry is none, expiry) so a null expiry_date is never
order-compared against a datetime.date, which raised typeerror in
python 3 when a warehouse held both dated and never-expiring batches.

erpnext/stock/doctype/batch/batch.py

@@ -297,7 +297,7 @@ def get_batches_by_oldest(item_code: str, warehouse: str):
 	"""Returns the oldest batch and qty for the given item_code and warehouse"""
 	batches = get_batch_qty(item_code=item_code, warehouse=warehouse)
 	batches_dates = [[batch, frappe.get_value("Batch", batch.batch_no, "expiry_date")] for batch in batches]
-	batches_dates.sort(key=lambda tup: tup[1])
+	batches_dates.sort(key=lambda tup: (tup[1] is None, tup[1]))
 	return batches_dates