wp query meta value is not null

Regarding your first problem, because $args is an PHP: array you could do something like this:

Code:

$args = array(
    'orderby'          => 'post_date',
    'order'             => 'DESC',
    'post_type'        => 'custom_post_type',
    'post_status'      => 'publish',
    'posts_per_page' => '150'
);

if ( $catArr ) {
    $args['tax_query'] = array(
      array(
        'taxonomy' => 'tax_type',
        'field' => 'id',
        'terms' => $catArr
      )
    );
}

Note: code is exemplary

Regarding your second question, regarding the meta_query, you should take a look at this:

• Check for the existence of custom field