ensures double negative is evaluated correctly

[hedyyytang]

what's changed
Update the negate function in BinaryOperatorExpression to negate the existing value. This allows expressions with double negative, such as NOT (:gameboy IS NOT NULL), to negate correctly.

Example
Prior to the change, the following query will return count 16 because NOT ("gameboy" IS NOT NULL) is evaluated to TRUE and making the whole WHERE clause always TRUE. The expression, NOT ("gameboy" IS NOT NULL) should be FALSE because if "gameboy" IS NOT NULL is true, then NOT(TRUE) is FALSE

$query = $pdo->prepare("SELECT COUNT(*) as 'count' FROM `video_game_characters` WHERE (:console IS NOT NULL AND `console` = :console) OR NOT (:console IS NOT NULL)");
        $query->bindValue(':console', 'gameboy');
        $query->execute();

After the change, the query returns 1 correctly.

Tests
Add tests with queries involving IS NULL AND IS NOT NULL