Replace ”No categories”

alternative approach:

using a filter function for 'widget_categories_args' to change the 'show_option_none' string of the wp_list_categories() function used by the widget;

code to be added to functions.php of your (child) theme:

add_filter( 'widget_categories_args', 'custom_no_categories_text' );
function custom_no_categories_text( $cat_args ) {
	$cat_args['show_option_none'] = 'Sorry nothing found';
	return $cat_args;
}

https://codex.wordpress.org/Plugin_API/Filter_Reference/widget_categories_args
https://developer.wordpress.org/reference/functions/wp_list_categories/