[SPARK-20359][SQL] Avoid unnecessary execution in EliminateOuterJoin...

[SPARK-20359][SQL] Avoid unnecessary execution in EliminateOuterJoin optimization that can lead to NPE

Avoid necessary execution that can lead to NPE in EliminateOuterJoin and add test in DataFrameSuite to confirm NPE is no longer thrown

## What changes were proposed in this pull request?
Change leftHasNonNullPredicate and rightHasNonNullPredicate to lazy so they are only executed when needed.

## How was this patch tested?

Added test in DataFrameSuite that failed before this fix and now succeeds. Note that a test in catalyst project would be better but i am unsure how to do this.

Please review http://spark.apache.org/contributing.html

 before opening a pull request.

Author: Koert Kuipers <koert@tresata.com>

Closes #17660 from koertkuipers/feat-catch-npe-in-eliminate-outer-join.

(cherry picked from commit 608bf30f)
Signed-off-by: Wenchen Fan <wenchen@databricks.com>