[SPARK-21826][SQL] outer broadcast hash join should not throw NPE

## What changes were proposed in this pull request?

This is a bug introduced by https://github.com/apache/spark/pull/11274/files#diff-7adb688cbfa583b5711801f196a074bbL274 .

Non-equal join condition should only be applied when the equal-join condition matches.

## How was this patch tested?

regression test

Author: Wenchen Fan <wenchen@databricks.com>

Closes #19036 from cloud-fan/bug.