[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.