[SPARK-1914] [SQL] Simplify CountFunction not to traverse to evaluate all child expressions.

`CountFunction` should count up only if the child's evaluated value is not null.

Because it traverses to evaluate all child expressions, even if the child is null, it counts up if one of the all children is not null.

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

Closes #861 from ueshin/issues/SPARK-1914 and squashes the following commits:

3b37315 [Takuya UESHIN] Merge branch 'master' into issues/SPARK-1914
2afa238 [Takuya UESHIN] Simplify CountFunction not to traverse to evaluate all child expressions.

(cherry picked from commit d6395d86f90d1c47c5b6ad17c618b56e00b7fc85)
Signed-off-by: Reynold Xin <rxin@apache.org>