PHP reproduce exact working sql query but no result - php

Question

I've got a stored procedure which looks like this:
-- GET INVITES
DROP PROCEDURE IF EXISTS pda.get_invite;
DELIMITER //
CREATE PROCEDURE pda.get_invite ( code varchar(100), invitator_id bigint )
BEGIN
SELECT * FROM pda_invites
WHERE (pda_invites.invitator_id = invitator_id || invitator_id IS NULL)
AND (pda_invites.code = code || code IS NULL);
END //
CALL get_invite ( 'cec95191-23db-11e9-86d7-26374278e97f', NULL );
That works in phpmyadmin and deliver the desired result.
Now on php side, I've got some kind of query builder in a procedure class with registered arguments, which looks like this:
public function toSQL ( ) {
// Make sure params are set
if ( $this->ready == false ) {
throw new Exception ( 'Called unready procedure ' . $this->PROCEDURE_NAME . '.' );
return false;
}
// Build query
$sql= 'CALL ' . $this->PROCEDURE_NAME . '( ';
$i = 0;
foreach ( $this->args as $arg ) {
$param = $this->params[ $i ][ $arg['NAME'] ];
// Apply valid null values
if ( $param == null || $param == 'null' ) {
$sql = $sql . 'NULL';
// Apply varchar strings
} else if ( $arg['TYPE'] == 'varchar' ) {
$sql = $sql . '\'' . $param . '\'';
// Apply numeric values
} else {
$sql = $sql . $param;
}
if ( ($i+1) < $this->args_count ) {
$sql = $sql . ', ';
}
$i++;
}
$sql = $sql . ' );';
return $sql;
}
In my DB Service I trigger that by calling:
$sql = $procedure->toSQL();
$result = $con->query($sql);
echo($sql);
echo json_encode($result);
That results in following equivalent query string which is also reflected by the mysql log:
CALL get_invite( 'cec95191-23db-11e9-86d7-26374278e97f', NULL );
But the problem and question is that it produces an empty result:
{"current_field":null,"field_count":null,"lengths":null,"num_rows":null,"type":null}
Have anyone a clue why it coulds behave this way?