minor fixes in the assessment result tool (#11795)

erpnext/education/doctype/assessment_result_tool/assessment_result_tool.js

@@ -31,6 +31,7 @@ frappe.ui.form.on('Assessment Result Tool', {
 					"student_group": frm.doc.student_group
 				},
 				callback: function(r) {
+					if (r.message) {
 						frm.doc.students = r.message;
 						frm.events.render_table(frm);
 						for (let value of r.message) {
@@ -41,6 +42,7 @@ frappe.ui.form.on('Assessment Result Tool', {
 						}
 						frm.events.submit_result(frm);
 					}
+				}
 			});
 		}
 	},