You need to make the whole line conditional, like this:
$sub_field_title = get_sub_field('title');
if (!empty($sub_field_title)) {
echo '<h1 class="title">' . $sub_field_title . '</h1>';
}
You need to make the whole line conditional, like this:
$sub_field_title = get_sub_field('title');
if (!empty($sub_field_title)) {
echo '<h1 class="title">' . $sub_field_title . '</h1>';
}