[SPARK-21726][SQL][FOLLOW-UP] Check for structural integrity of the plan in Optimzer in test mode

## What changes were proposed in this pull request?

The condition in `Optimizer.isPlanIntegral` is wrong. We should always return `true` if not in test mode.

## How was this patch tested?

Manually test.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #19161 from viirya/SPARK-21726-followup.