Minor Description
Since https://github.com/apache/spark/pull/19825 PropagateEmptyRelation has been taught to handle more cases it can cause the optimized query plan to be unresolved.
Simple repro is to run following through the optimizer
LocalRelation.fromExternalRows(Seq('a.int), data = Seq(Row(1))) .join(LocalRelation('a.int, 'b.int), UsingJoin(FullOuter, "a" :: Nil), None)
Which results in
Project [coalesce(a#0, null) AS a#7, null AS b#6] +- LocalRelation [a#0]
This then fails type check on coalesce expression since `a` and null have different type.
Simple, targeted fix is to change PropagateEmptyRelation to add casts around nulls. More comprehensive fix would be to run type coercion at the end of optimization so it can fix cases like those. Alternatively the type checking code could treat NullType as equal to any other type and not fail the type check in the first place.
