Fix "addParticipantOnce" store action
Fix "addParticipantOnce" store action

Due to the missing reference to the getters the participant index was
not properly got, so even if the participant did not exist it was not
"-1" and thus the participant was never added.

Signed-off-by: Daniel Calviño Sánchez <bd7629e975cc0e00ea7cda4ab6a120f2ee42ac1b@gmail.com>

nextcloud/spreed,nextcloud/spreed,nextcloud/spreed