fix: fetch rate only if field is empty

erpnext/hr/doctype/expense_taxes_and_charges/expense_taxes_and_charges.json

@@ -54,6 +54,7 @@
   {
    "columns": 2,
    "fetch_from": "account_head.tax_rate",
+   "fetch_if_empty": 1,
    "fieldname": "rate",
    "fieldtype": "Float",
    "in_list_view": 1,
@@ -92,7 +93,7 @@
   }
  ],
  "istable": 1,
- "modified": "2019-06-19 17:47:40.236436",
+ "modified": "2019-06-20 12:01:33.919555",
  "modified_by": "Administrator",
  "module": "HR",
  "name": "Expense Taxes and Charges",