Add this filter to the query.
Due to the nature of fapi, the value and the operator have an unintended level of indirection. You will find them in $this->operator and $this->value respectively.
Overrides BooleanOperator::query
public function query() {
$this
->ensureMyTable();
$where = "{$this->tableAlias}.{$this->realField} ";
if (empty($this->value)) {
$where .= "= ''";
if ($this->accept_null) {
$where = '(' . $where . " OR {$this->tableAlias}.{$this->realField} IS NULL)";
}
}
else {
$where .= "<> ''";
}
$this->query
->add_where($this->options['group'], $where);
}