[SPARK-2036] [SQL] CaseConversionExpression should check if the evaluated value is null.

`CaseConversionExpression` should check if the evaluated value is `null`.

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

Closes #982 from ueshin/issues/SPARK-2036 and squashes the following commits:

61e1c54 [Takuya UESHIN] Add check if the evaluated value is null.

(cherry picked from commit e4c11eef2f64df0b6a432f40b669486d91ca6352)
Signed-off-by: Michael Armbrust <michael@databricks.com>