#wp-admin-bar-lap-websites .ab-item.ab-empty-item {
  /* background-image: url("path/to/your/icon.svg") no-repeat; */
  background-size: 20px 20px;
  background-position: 2px 5px;
  padding-left: 28px;
}
#wp-admin-bar-lap-websites .ab-item.ab-empty-item:before{
content: "";
font-family: "dashicons";
}