[FIX] web: FormView with OuterGroup empty

Before this commit, if a form view has a group containing only invisible
elements and at least one invisible group, then the view crashes.

Example:
    <form>
        <sheet>
            <group>
                <group invisible="1">
                    <field name="foo"/>
                </group>
            </group>
        </sheet>
    </form>

The solution is to have an empty list as the default props for the
OuterGroup slots. OuterGroup is a group containing at least one other group.

closes odoo/odoo#97295

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>

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

@@ -31,6 +31,10 @@ export class OuterGroup extends Group {
     }
 }
 OuterGroup.template = "web.Form.OuterGroup";
+OuterGroup.defaultProps = {
+    ...Group.defaultProps,
+    slots: [],
+};
 
 export class InnerGroup extends Group {
     getRows() {

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

@@ -299,6 +299,27 @@ QUnit.module("Views", (hooks) => {
         assert.containsNone(target, "label.o_form_label_empty:contains(timmy)");
     });
 
+    QUnit.test("form view with a group that contains an invisible group", async function (assert) {
+        await makeView({
+            type: "form",
+            resModel: "partner",
+            serverData,
+            arch: `
+                <form>
+                    <sheet>
+                        <group>
+                            <group invisible="1">
+                                <field name="foo"/>
+                            </group>
+                        </group>
+                    </sheet>
+                </form>`,
+            resId: 2,
+        });
+
+        assert.containsOnce(target, ".o_form_view .o_group");
+    });
+
     QUnit.test("status bar rendering without buttons", async (assert) => {
         await makeView({
             type: "form",