[SPARK-1915] [SQL] AverageFunction should not count if the evaluated value is null.

Average values are difference between the calculation is done partially or not partially.
Because `AverageFunction` (in not-partially calculation) counts even if the evaluated value is null.

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

Closes #862 from ueshin/issues/SPARK-1915 and squashes the following commits:

b1ff3c0 [Takuya UESHIN] Modify AverageFunction not to count if the evaluated value is null.