Add empty default for jitsi-prosody modules

roles/matrix-jitsi/defaults/main.yml

@@ -39,6 +39,7 @@ matrix_jitsi_xmpp_bosh_url_base: http://{{ matrix_jitsi_xmpp_server }}:5280
 matrix_jitsi_xmpp_guest_domain: guest.meet.jitsi
 matrix_jitsi_xmpp_muc_domain: muc.meet.jitsi
 matrix_jitsi_xmpp_internal_muc_domain: internal-muc.meet.jitsi
+matrix_jitsi_xmpp_modules: ''
 
 matrix_jitsi_recorder_domain: recorder.meet.jitsi