test: refactor test to spy on Object.entries

packages/charts/test/private-api.test.js

@@ -1,5 +1,6 @@
 import { expect } from '@esm-bundle/chai';
 import { fixtureSync, oneEvent } from '@vaadin/testing-helpers';
+import sinon from 'sinon';
 import '../vaadin-chart.js';
 import { inflateFunctions } from '../src/helpers.js';
 
@@ -34,9 +35,16 @@ describe('vaadin-chart private API', () => {
     });
 
     it('should not try to inflate if a non-object value is passed', () => {
+      const spy = sinon.spy(Object, 'entries');
       inflateFunctions(1);
+      expect(spy.callCount).to.be.equal(0);
       inflateFunctions(null);
-      inflateFunctions([1]);
+      expect(spy.callCount).to.be.equal(0);
+      const arr = [1, 'a'];
+      inflateFunctions(arr);
+      expect(spy.callCount).to.be.equal(0);
+      inflateFunctions({});
+      expect(spy.callCount).to.be.equal(1);
     });
   });