fixed failing test. (#25619)

public/app/plugins/panel/graph/specs/graph_ctrl.test.ts

@@ -91,7 +91,7 @@ describe('GraphCtrl', () => {
     });
     });
 
 
     it('should set datapointsOutside', () => {
     it('should set datapointsOutside', () => {
-      expect(ctx.ctrl.dataWarning).toBe(null);
+      expect(ctx.ctrl.dataWarning).toBeUndefined();
     });
     });
   });
   });