IMPALA-1016: Return correct number of NULL values when projecting newly added column

This patch handles the case where when a query was projecting a newly added column,
the parquet scanner was returning infinite values.

Change-Id: Ie5f4d4a88d5868e8d9e5c39fa9440821776dde3c
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2725
Reviewed-by: Marcel Kornacker <marcel@cloudera.com>
Tested-by: jenkins