[maddinat0r]

'0' and 'NULL' aren't the same values. A numeric field can be '0' or 'NULL' (if allowed), it would destroy the whole point of 'NULL's existence if they were both treated as one and the same. The plugin and the MySQL C API treat '0' as a number and 'NULL' as a string, that's why you can't use cache_get_*_int with 'NULL' as value.
Take a look at the MySQL documentation here and this site and you'll see why it doesn't make sense to use 'NULL' as a numeric value.