[SPARK-4319][SQL] Enable an ignored test "null count".

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #3185 from ueshin/issues/SPARK-4319 and squashes the following commits:

a44a38e [Takuya UESHIN] Enable an ignored test "null count".

(cherry picked from commit dbf10588de03e8ea993fff687a78727eff55db1f)
Signed-off-by: Michael Armbrust <michael@databricks.com>