fix(Timesheet): fetch exchange rate only if currency is set (#30057)

diff --git a/erpnext/projects/doctype/timesheet/timesheet.js b/erpnext/projects/doctype/timesheet/timesheet.js
index f615f05..453d46c 100644
--- a/erpnext/projects/doctype/timesheet/timesheet.js
+++ b/erpnext/projects/doctype/timesheet/timesheet.js
@@ -116,7 +116,7 @@
 
 	currency: function(frm) {
 		let base_currency = frappe.defaults.get_global_default('currency');
-		if (base_currency != frm.doc.currency) {
+		if (frm.doc.currency && (base_currency != frm.doc.currency)) {
 			frappe.call({
 				method: "erpnext.setup.utils.get_exchange_rate",
 				args: {