### What changes were proposed in this pull request?
This PR is to fix an incorrect outer join elimination when filter's `isNotNull` constraints is unable to filter out all null-supplying rows. For example, `isnotnull(coalesce(b#227, c#238))`.
Users can hit this error when they try to use `using/natural outer join`, which is converted to a normal outer join with a `coalesce` expression on the `using columns`. For example,
```Scala
val a = Seq((1, 2), (2, 3)).toDF("a", "b")
val b = Seq((2, 5), (3, 4)).toDF("a", "c")
val c = Seq((3, 1)).toDF("a", "d")
val ab = a.join(b, Seq("a"), "fullouter")
ab.join(c, "a").explain(true)
```
The dataframe `ab` is doing `using full-outer join`, which is converted to a normal outer join with a `coalesce` expression. Constraints inference generates a `Filter` with constraints `isnotnull(coalesce(b#227, c#238))`. Then, it triggers a wrong outer join elimination and generates a wrong result.
```
Project [a#251, b#227, c#237, d#247]
+- Join Inner, (a#251 = a#246)
:- Project [coalesce(a#226, a#236) AS a#251, b#227, c#237]
: +- Join FullOuter, (a#226 = a#236)
: :- Project [_1#223 AS a#226, _2#224 AS b#227]
: : +- LocalRelation [_1#223, _2#224]
: +- Project [_1#233 AS a#236, _2#234 AS c#237]
: +- LocalRelation [_1#233, _2#234]
+- Project [_1#243 AS a#246, _2#244 AS d#247]
+- LocalRelation [_1#243, _2#244]
== Optimized Logical Plan ==
Project [a#251, b#227, c#237, d#247]
+- Join Inner, (a#251 = a#246)
:- Project [coalesce(a#226, a#236) AS a#251, b#227, c#237]
: +- Filter isnotnull(coalesce(a#226, a#236))
: +- Join FullOuter, (a#226 = a#236)
: :- LocalRelation [a#226, b#227]
: +- LocalRelation [a#236, c#237]
+- LocalRelation [a#246, d#247]
```
**A note to the `Committer`**, please also give the credit to dongjoon-hyun who submitted another PR for fixing this issue. https://github.com/apache/spark/pull/14580
### How was this patch tested?
Added test cases
Author: gatorsmile <gatorsmile@gmail.com>
Closes#14661 from gatorsmile/fixOuterJoinElimination.
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