if ( $units === true ) {
  $term_ids = '';
  $unit_tax_query = [
    'taxonomy'  =>  'ctax_units'
  ];

  if ( ! empty( $atts['units'] ) ) {
    $unit_term_ids = explode(', ', $atts['units']);
    $i = 0;
    while ( $i < sizeof($unit_term_ids) ) {
      $unit_term_ids[$i] = (int) $unit_term_ids[$i];
      $i++;
    }
                
    $unit_tax_query['field']  = 'term_id';
    $unit_tax_query['terms']  = $unit_term_ids;
                
  }
        
}

// var_dumps
array(3) {
  ["taxonomy"]=>
  string(10) "ctax_units"
  ["field"]=>
  string(7) "term_id"
  ["terms"]=>
  array(2) {
    [0]=>
    int(24)
    [1]=>
    int(40)
  }
}