IMPALA-1468: Proper typing of NULL literals in InlineView.makeOutputNullable().

When creating the smap for substituting NULL literals for SlotRefs in
InlineView.makeOutputNullable() the NULL literals must have the same type
as the corresponding SlotRefs they replace to ensure that exprs resolve to
the same signature after applying the smap.

Change-Id: I8dc58c42279f15b8516d6385556fc8bb77fc7e51
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5283
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins