[FIX] mail: rtc session counting in updatertcsessions

Before this commit:

The logic used to count the number of RTC Sessions was incorrect. Which led to
multiple sounds being played sometimes. Etc, on start a meeting

After this commit:

Corrected the RTC Sessions counting logic.

closes odoo/odoo#140628

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

addons/mail/static/src/discuss/call/common/rtc_service.js

@@ -1526,7 +1526,7 @@ export class Rtc {
         if (!channel) {
             return;
         }
-        const oldCount = Object.keys(channel.rtcSessions).length;
+        const oldCount = channel.rtcSessions.length;
         channel.rtcSessions = sessionsData;
         if (channel.rtcSessions.length > oldCount) {
             this.soundEffectsService.play("channel-join");