[FIX] web: correct type of defaultPage prop

Steps to reproduce:
- go to "Link Tracker" app;
- go to "Campaigns" tab (UTMs);
- create a new campaign;
- leave and return in this record.

Issue:
A traceback appears.
Error message:
`Invalid props for component 'Notebook': 'defaultPage' is not a string`

Cause:
The `defaultPage` prop must be a string.
When the record is new, it will be `undefined`,
so it will pass props validation because this prop is optional.
But when we try to calculate it and it does not return a value,
the `defaultPage` is null.
The variable is therefore declared but it is empty
(and therefore not a string).
`null` value does not pass the props validation test.
The problem appears in the "Campaigns" tab
because there is a notebook without content.

Solution:
Test if it is possible to have a value.
If not, set the `defaultPage` prop to be `undefined`.

opw-3109327

closes odoo/odoo#109533

X-original-commit: 402b1a822c3983af33aa1d3a3c7e93d8836c60d8
Signed-off-by: Dardenne Florent (dafl) <dafl@odoo.com>

addons/web/static/src/views/form/form_controller.js

@@ -187,7 +187,9 @@ export class FormController extends Component {
         this.fieldsToTranslate = useState(fieldsToTranslate || {});
         const activeNotebookPages = { ...state.activeNotebookPages };
         this.onNotebookPageChange = (notebookId, page) => {
-            activeNotebookPages[notebookId] = page;
+            if (page) {
+                activeNotebookPages[notebookId] = page;
+            }
         };
 
         useSetupView({

addons/web/static/tests/views/form/form_view_tests.js

@@ -12976,4 +12976,41 @@ QUnit.module("Views", (hooks) => {
         assert.strictEqual(target.querySelector(".o_form_nosheet").innerHTML, "<div></div>");
         assert.verifySteps(["warn", "warn", "warn", "warn"]);
     });
+
+    QUnit.test("reload form view with an empty notebook", async function (assert) {
+        assert.expect(1);
+
+        serverData.views = {
+            "partner,false,form": `
+            <form>
+                <sheet>
+                    <notebook>
+                    </notebook>
+                </sheet>
+            </form>`,
+            "partner,false,list": '<tree><field name="foo"/></tree>',
+            "partner,false,search": "<search></search>",
+        };
+
+        serverData.actions = {
+            1: {
+                id: 1,
+                name: "Partner",
+                res_model: "partner",
+                type: "ir.actions.act_window",
+                views: [
+                    [false, "list"],
+                    [false, "form"],
+                ],
+            },
+        };
+
+        const target = getFixture();
+        const webClient = await createWebClient({ serverData });
+        await doAction(webClient, 1);
+        await click(target.querySelector(".o_data_row .o_data_cell"));
+        await click(target.querySelector(".o_back_button"));
+        await click(target.querySelector(".o_data_row .o_data_cell"));
+        assert.containsOnce(target, ".o_form_view");
+    });
 });