sperakerstats: prevent access of nil object (#5112)

If the dominant speaker leaves their object will be gone from the mapping.

resources/prosody-plugins/mod_speakerstats_component.lua

         local oldDominantSpeakerId = roomSpeakerStats['dominantSpeakerId'];
 
         if oldDominantSpeakerId then
-            roomSpeakerStats[oldDominantSpeakerId]:setDominantSpeaker(false);
+            local oldDominantSpeaker = roomSpeakerStats[oldDominantSpeakerId];
+            if oldDominantSpeaker then
+                oldDominantSpeaker:setDominantSpeaker(false);
+            end
         end
 
         if newDominantSpeaker then