[SPARK-14986][SQL] Return correct result for empty LATERAL VIEW OUTER

## What changes were proposed in this pull request?
A Generate with the `outer` flag enabled should always return one or more rows for every input row. The optimizer currently violates this by rewriting `outer` Generates that do not contain columns of the child plan into an unjoined generate, for example:
```sql
select e from a lateral view outer explode(a.b) as e
```
The result of this is that `outer` Generate does not produce output at all when the Generators' input expression is empty. This PR fixes this.

## How was this patch tested?
Added test case to `SQLQuerySuite`.

Author: Herman van Hovell <hvanhovell@questtec.nl>

Closes #12906 from hvanhovell/SPARK-14986.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -362,7 +362,8 @@ object ColumnPruning extends Rule[LogicalPlan] {
       g.copy(child = prunedChild(g.child, g.references))
 
     // Turn off `join` for Generate if no column from it's child is used
-    case p @ Project(_, g: Generate) if g.join && p.references.subsetOf(g.generatedSet) =>
+    case p @ Project(_, g: Generate)
+        if g.join && !g.outer && p.references.subsetOf(g.generatedSet) =>
       p.copy(child = g.copy(join = false))
 
     // Eliminate unneeded attributes from right side of a Left Existence Join.

sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala

@@ -2473,4 +2473,11 @@ class SQLQuerySuite extends QueryTest with SharedSQLContext {
           Row("r3c1x", "r3c2", "t1r3c3", "r3c2", "t1r3c3") :: Nil)
     }
   }
+
+  test("SPARK-14986: Outer lateral view with empty generate expression") {
+    checkAnswer(
+      sql("select nil from (select 1 as x ) x lateral view outer explode(array()) n as nil"),
+      Row(null) :: Nil
+    )
+  }
 }