Update stock_entry.py

fixed for null expense account
diff --git a/erpnext/stock/doctype/stock_entry/stock_entry.py b/erpnext/stock/doctype/stock_entry/stock_entry.py
index 051d9fc..ea5c374 100644
--- a/erpnext/stock/doctype/stock_entry/stock_entry.py
+++ b/erpnext/stock/doctype/stock_entry/stock_entry.py
@@ -100,7 +100,7 @@
 					if f in ["stock_uom", "conversion_factor"] or not item.get(f):
 						item.set(f, item_details.get(f))
 
-			if self.difference_account:
+			if self.difference_account and item.expense_account=="":
 				item.expense_account = self.difference_account
 
 			if not item.transfer_qty: