[SPARK-20534][SQL] Make outer generate exec return empty rows

## What changes were proposed in this pull request?
Generate exec does not produce `null` values if the generator for the input row is empty and the generate operates in outer mode without join. This is caused by the fact that the `join=false` code path is different from the `join=true` code path, and that the `join=false` code path did deal with outer properly. This PR addresses this issue.

## How was this patch tested?
Updated `outer*` tests in `GeneratorFunctionSuite`.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #17810 from hvanhovell/SPARK-20534.