Allow undefined values in CQL query args and translate them to NULL

Rather than checking for undefined values in the args array, check that the index exists, and treat undefined values as nulls.

Signed-off-by: Daniel Smedegaard Buus <danielbuus@gmail.com>
Merge pull request #49 from DanielSmedegaardBuus/master

Allow undefined values in CQL query args and translate them to NULL