Always fallback to the guest avatar instead of none

Signed-off-by: Joas Schilling <coding@schilljs.com>

app/src/main/java/com/nextcloud/talk/models/json/chat/ChatMessage.java

@@ -285,7 +285,7 @@ public class ChatMessage implements MessageContentType, MessageContentType.Image
             public String getAvatar() {
                 if (getActorType().equals("users")) {
                     return ApiUtils.getUrlForAvatarWithName(getActiveUser().getBaseUrl(), actorId, R.dimen.avatar_size);
-                } else if (getActorType().equals("guests")) {
+                } else {
                     String apiId =
                             NextcloudTalkApplication.Companion.getSharedApplication().getString(R.string.nc_guest);
 
@@ -293,8 +293,6 @@ public class ChatMessage implements MessageContentType, MessageContentType.Image
                         apiId = getActorDisplayName();
                     }
                     return ApiUtils.getUrlForAvatarWithNameForGuests(getActiveUser().getBaseUrl(), apiId, R.dimen.avatar_size);
-                } else {
-                    return null;
                 }
             }
         };