COMPUTE INCREMENTAL STATS sets num_nulls off-by-one

IMPALA-7659 added the population of NULL counts while computing stats, but this functionality isn't working properly for incremental stats. The query is produced correctly, but the null count set in the table is one lower than it should be. In the case that the table has no nulls, this ends up setting a '-1' count, which is interpreted as 'unknown'. In the case that there are nulls, we'll just be a little inaccurate.