[FIX] mail: prevents crash in some channelMember inserts

Before this commit, we were trying to access
`channelMember.thread.channelMembers` when inserting a member, but
some inserts come without this key.

A traceback could occur when inserts of `rtcSession` records came
with a `channelMember` with little information on their thread.

closes odoo/odoo#116656

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/mail/static/src/core/channel_member_service.js

@@ -37,7 +37,7 @@ export class ChannelMemberService {
         switch (command) {
             case "insert":
                 {
-                    if (!member.thread.channelMembers.includes(member)) {
+                    if (!member.thread?.channelMembers?.includes(member)) {
                         member.thread.channelMembers.push(member);
                     }
                 }