Always initialize PartitionBoundInfoData's null_index.

This doesn't actually matter at present, because the current code
never consults null_index for range partitions.  However, leaving
it uninitialized is still a bad idea, so let's not do that.

Amul Sul, reviewed by Ashutosh Bapat

Discussion: http://postgr.es/m/CAAJ_b94AkEzcx+12ySCnbMDX7=UdF4BjnoBGfMQbB0RNSTo3Ng@mail.gmail.com