fix verify_app_name to bomb on null app. use it in nginx:build-config

plugins/common/functions

@@ -104,6 +104,7 @@ dokku_container_log_verbose_quiet() {
 
 verify_app_name() {
   local APP="$1"
+  [[ ! -n "$APP" ]] && dokku_log_fail "(verify_app_name) APP must not be null"
   [[ ! -d "$DOKKU_ROOT/$APP" ]] && dokku_log_fail "App $APP does not exist"
   IMAGE="dokku/$APP"
   return 0

plugins/nginx-vhosts/commands

@@ -28,6 +28,7 @@ restart_nginx () {
 case "$1" in
   nginx:build-config)
     APP="$2"; DOKKU_APP_LISTEN_PORT="$3"; DOKKU_APP_LISTEN_IP="${4}"
+    verify_app_name "$APP"
     VHOST_PATH="$DOKKU_ROOT/$APP/VHOST"
     URLS_PATH="$DOKKU_ROOT/$APP/URLS"
     WILDCARD_SSL="$DOKKU_ROOT/tls"