Avoid error when testing through jest about removing event listenner

Avoid:
```
  TypeError: Cannot read properties of null (reading 'removeEventListener')
        at extended.disconnect (/home/runner/work/openfoodnetwork/openfoodnetwork/app/webpacker/controllers/dropdown_controller.js:32:14)
        at Context.disconnect (/home/runner/work/openfoodnetwork/openfoodnetwork/node_modules/stimulus/dist/stimulus.umd.js:1441:25)
```

spec/javascripts/stimulus/dropdown_controller_test.js

@@ -23,6 +23,10 @@ describe("Dropdown controller", () => {
       </div>`;
     });
 
+    afterEach(() => {
+      document.body.innerHTML = "";
+    });
+
     it("hide menu by default", () => {
       const menu = document.getElementById("menu");
       expect(menu.classList.contains("hidden")).toBe(true);