fix: Allow KID index '0'

[ajupazhamayil]

Fixes #504

[ajupazhamayil]

Note:

php > $nullValue = null;
php > $zero = '0';
php > $text = 'test';
php > print(!isset($nullValue) . PHP_EOL . !isset($zero) . PHP_EOL . !isset($text) . PHP_EOL);
1


php > print(empty($nullValue) . PHP_EOL . empty($zero) . PHP_EOL . empty($text) . PHP_EOL);
1
1