Fix to avoid putting an incorrect empty jitsi.preferredDomain when no local jitsi

roles/matrix-riot-web/templates/config.json.j2

@@ -30,7 +30,7 @@
 	"embeddedPages": {
 		"homeUrl": {{ matrix_riot_web_embedded_pages_home_url|string|to_json }}
 	},
-	{% if matrix_riot_web_jitsi_preferredDomain is not none %}
+	{% if matrix_riot_web_jitsi_preferredDomain is not none and matrix_riot_web_jitsi_preferredDomain != '' %}
 	"jitsi": {
         "preferredDomain": {{ matrix_riot_web_jitsi_preferredDomain|to_json }}
     },