[SPARK-18442][SQL] Fix nullability of WrapOption.

## What changes were proposed in this pull request?

The nullability of `WrapOption` should be `false`.

## How was this patch tested?

Existing tests.

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

Closes #15887 from ueshin/issues/SPARK-18442.

(cherry picked from commit 170eeb34)
Signed-off-by: Wenchen Fan <wenchen@databricks.com>