How to modify query on category pages to show only sticky posts?

Question

I'm using the following code to filter posts on category pages so only sticky posts are displayed.

add_action( 'pre_get_posts', function( \WP_Query $q ) {
if( ! is_admin() && $q->is_category() && $q->is_main_query() ) {
    $sticky_posts = get_option( 'sticky_posts' );
    if( ! empty( $sticky_posts ) )
        $q->set( 'post__in', (array) $sticky_posts );
}
} );
?>

The problem is, if there are no sticky posts then all the category's posts are displayed. If there are no sticky posts I don't want any posts to be displayed.

Answer 1

Just set 'post__in' to a null array.

add_action( 'pre_get_posts', function( \WP_Query $q ) {
if( ! is_admin() && $q->is_category() && $q->is_main_query() ) {
    $sticky_posts = get_option( 'sticky_posts' );
    //If there are sticky posts
    if( ! empty( $sticky_posts ) ) {
        $q->set( 'post__in', (array) $sticky_posts );
    }
    //If not
    else {
        $q->set( 'post__in', array(0) );
    }
}
} );

Answer 2

Use else statement for the no result found

<?php 
add_action( 'pre_get_posts', function( \WP_Query $q ) {
if( ! is_admin() && $q->is_category() && $q->is_main_query() ) {
    $sticky_posts = get_option( 'sticky_posts' );
    if( ! empty( $sticky_posts ) ) {
        $q->set( 'post__in', (array) $sticky_posts );
} else {

    echo "No posts found";

}
}
} );
?>