This icon is defined using CSS. You can override it using ".scheduler_default_tree_image_no_children" selector (for the default CSS theme):
.scheduler_default_tree_image_no_children {
background-image: none !important;
}
In order to override the default theme, use "!important" or a more specific selector like "#dp .scheduler_default_tree_image_no_children".
You can also generate your own theme using the theme designer and remove the *_tree_image_no_children definition from the generated CSS file:
https://themes.daypilot.org/scheduler/create