Do echo inside an if statement.
function show_my_message() {
$settings = get_option('wc_checkout_message_settings');
//Check if the array is set
if( isset($settings['wc_checkout_message_text_field_0']) && !empty($settings['wc_checkout_message_text_field_0']) ){
echo $settings['wc_checkout_message_text_field_0'];
}
//Check the other value and show it
if(isset($settings['wc_checkout_message_text_field_1']) && !empty($settings['wc_checkout_message_text_field_1']) ){
echo $settings['wc_checkout_message_text_field_1'];
}
}