Enabled NULL values in prepared statements.

So far, users cannot send a NULL value among the parameters of prepared statement. This update should enable sending NULL values by push-backing them to the binary stream of parameters. However, to carry the NULL value, the parameter container has to store pointers instead of the actual values. User will now be able to push_back_null(), the NULL pointer will be carried in the container and checked when encoding to be sent as the appropriate binary representation of NULL (inspired here by the corresponding part of code in the Java driver).