Allow for 0 valuation rate in Stock Reconciliation (#15313)

Currently, a valuation rate of 0 gets treated the same as an omitted
valuation rate, and gets overwritten during validation. This hotfix
allows a Stock Reconciliation Item's valuation rate to be set to 0.
diff --git a/erpnext/stock/doctype/stock_reconciliation/stock_reconciliation.py b/erpnext/stock/doctype/stock_reconciliation/stock_reconciliation.py
index 5e69664..5b6e706 100644
--- a/erpnext/stock/doctype/stock_reconciliation/stock_reconciliation.py
+++ b/erpnext/stock/doctype/stock_reconciliation/stock_reconciliation.py
@@ -109,7 +109,7 @@
 				self.validation_messages.append(_get_msg(row_num,
 					_("Negative Valuation Rate is not allowed")))
 
-			if row.qty and not row.valuation_rate:
+			if row.qty and row.valuation_rate in ["", None]:
 				row.valuation_rate = get_stock_balance(row.item_code, row.warehouse,
 							self.posting_date, self.posting_time, with_valuation_rate=True)[1]
 				if not row.valuation_rate: