[SPARK-16181][SQL] outer join with isNull filter may return wrong result

## What changes were proposed in this pull request?

The root cause is: the output attributes of outer join are derived from its children, while they are actually different attributes(outer join can return null).

We have already added some special logic to handle it, e.g. `PushPredicateThroughJoin` won't push down predicates through outer join side, `FixNullability`.

This PR adds one more special logic in `FoldablePropagation`.

## How was this patch tested?

new test in `DataFrameSuite`

Author: Wenchen Fan <wenchen@databricks.com>

Closes #13884 from cloud-fan/bug.