[SPARK-18585][SQL] Use `ev.isNull = "false"` if possible for Janino to have a chance to optimize.

## What changes were proposed in this pull request?

Janino can optimize `true ? a : b` into `a` or `false ? a : b` into `b`, or if/else with literal condition, so we should use literal as `ev.isNull` if possible.

## How was this patch tested?

Existing tests.

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

Closes #16008 from ueshin/issues/SPARK-18585.