[FIX] mail: fold discuss app category when no conversation selected

Before this commit, when no conversation was selected in the
Discuss app, folding a category such as "Channels" results in
a crash.

This happens because `store.discuss.thread` is `undefined`, so it
must be properly guarded in the template of discuss sidebar category.

closes odoo/odoo#150828

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

addons/mail/static/src/discuss/core/web/discuss_sidebar_categories.xml

@@ -34,7 +34,7 @@
                 <t t-set="thread" t-value="thread"/>
             </t>
         </t>
-        <t t-elif="store.discuss.thread.in(category.threads)" t-call="mail.DiscussSidebarChannel">
+        <t t-elif="store.discuss.thread?.in(category.threads)" t-call="mail.DiscussSidebarChannel">
             <t t-set="thread" t-value="store.discuss.thread"/>
         </t>
     </t>

addons/mail/static/tests/discuss/core/web/discuss_tests.js

@@ -199,4 +199,7 @@ QUnit.test("no conversation selected when opening non-existing channel in discus
     const { openDiscuss } = await start();
     await openDiscuss(200); // non-existing id
     await contains("h4", { text: "No conversation selected." });
+    await contains(".o-mail-DiscussSidebarCategory-channel .oi-chevron-down");
+    await click(".o-mail-DiscussSidebar .btn", { text: "Channels" }); // check no crash
+    await contains(".o-mail-DiscussSidebarCategory-channel .oi-chevron-right");
 });