RegionScanExec reported null_count = 0 and distinct_count = total_rows for
every column of an append-only table. DataFusion answers count(col) and
count(DISTINCT col) from these statistics, so both returned the row count.
Keep the exact row count and an exact zero null count for non-nullable
columns, so count(*) is still answered from statistics.
Signed-off-by: Dennis Zhuang <killme2008@gmail.com>