[SPARK-18634][SQL][TRIVIAL] Touch-up Generate

## What changes were proposed in this pull request?
I jumped the gun on merging https://github.com/apache/spark/pull/16120, and missed a tiny potential problem. This PR fixes that by changing a val into a def; this should prevent potential serialization/initialization weirdness from happening.

## How was this patch tested?
Existing tests.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #16170 from hvanhovell/SPARK-18634.