[FIX] web: display_name "New" fallback should only target new record.

There is a fallback in the control panel
(https://github.com/odoo/odoo/blob/c22cb6bbadd38ecb050f6d4dc14672868beda20b/addons/web/static/src/search/control_panel/control_panel.xml#L137)
if the display_name of the record is empty.
But it is never actually used because there is another
fallback in the form_controller ("New"). This latter fallback
should only be used for new records, not for existing ones.

closes odoo/odoo#138061

Signed-off-by: Raphael Collet <rco@odoo.com>
Signed-off-by: Rémy Voet (ryv) <ryv@odoo.com>

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

@@ -333,7 +333,7 @@ export class FormController extends Component {
     }
 
     displayName() {
-        return this.model.root.data.display_name || _t("New");
+        return this.model.root.data.display_name || (this.model.root.isNew && _t("New")) || "";
     }
 
     async onPagerUpdate({ offset, resIds }) {

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

@@ -14212,4 +14212,16 @@ QUnit.module("Views", (hooks) => {
             assert.verifySteps(["web_read", "web_save"]);
         }
     );
+
+    QUnit.test("existing record with falsy display_name", async function (assert) {
+        serverData.models.partner.records[0].display_name = "";
+        await makeView({
+            type: "form",
+            resModel: "partner",
+            serverData,
+            arch: `<form><field name="int_field"/></form>`,
+            resId: 1,
+        });
+        assert.strictEqual(target.querySelector(".o_breadcrumb").innerText, "Unnamed");
+    });
 });