if statement in wp_query arguments

As said here : Conditional arguments for WP_Query and tax_query depending on if $somevar has a value

I can define the args outside of the WP_Query instantiation:

<?php
$tax_query = array('relation' => 'AND');
    if (isset($search_course_area))
    {
        $tax_query[] =  array(
                'taxonomy' => 'course-area',
                'field' => 'id',
                'terms' => $search_course_area
            );
    }
    if (isset($search_course_level))
    {
        $tax_query[] =  array(
                'taxonomy' => 'study-levels',
                'field' => 'id',
                'terms' => $search_course_level
            );
    }
    if (isset($search_course_mode))
    {
        $tax_query[] =  array(
                'taxonomy' => 'course-mode',
                'field' => 'id',
                'terms' => $search_course_mode
            );
    }

$query = new WP_Query(
    array(
        //Retreive ALL course posts
        'post_type' => 'course',
        'posts_per_page' => '-1',
        //Filter taxonomies by id's passed from course finder widget
        'tax_query' => $tax_query,
    )
);
?>