Fix potential crash in payment_entry.py (#15411)

The new code for cost center checking in payment_entry.py crashes if `voucher_type` is `None`, which is an explicit possibility if party type is Employee. This commit adds a simple check to make sure that voucher_type is not None before running `get_doc`.
diff --git a/erpnext/accounts/doctype/payment_entry/payment_entry.py b/erpnext/accounts/doctype/payment_entry/payment_entry.py
index 6c814ad..6a361a4 100644
--- a/erpnext/accounts/doctype/payment_entry/payment_entry.py
+++ b/erpnext/accounts/doctype/payment_entry/payment_entry.py
@@ -589,10 +589,11 @@
 		voucher_type = None
 
 	# Add cost center condition
-	doc = frappe.get_doc({"doctype": voucher_type})
-	condition = ""
-	if doc and hasattr(doc, 'cost_center'):
-		condition = " and cost_center='%s'" % cost_center
+	if voucher_type:
+		doc = frappe.get_doc({"doctype": voucher_type})
+		condition = ""
+		if doc and hasattr(doc, 'cost_center'):
+			condition = " and cost_center='%s'" % cost_center
 
 	orders = []
 	if voucher_type: