[SPARK-16582][SQL] Explicitly define isNull = false for non-nullable expressions

## What changes were proposed in this pull request?

This patch is just a slightly safer way to fix the issue we encountered in https://github.com/apache/spark/pull/14168 should this pattern re-occur at other places in the code.

## How was this patch tested?

Existing tests. Also, I manually tested that it fixes the problem in SPARK-16514 without having the proposed change in https://github.com/apache/spark/pull/14168

Author: Sameer Agarwal <sameerag@cs.berkeley.edu>

Closes #14227 from sameeragarwal/codegen.