[SPARK-22206][SQL][SPARKR] gapply in R can't work on empty grouping columns

## What changes were proposed in this pull request?

Looks like `FlatMapGroupsInRExec.requiredChildDistribution` didn't consider empty grouping attributes. It should be a problem when running `EnsureRequirements` and `gapply` in R can't work on empty grouping columns.

## How was this patch tested?

Added test.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #19436 from viirya/fix-flatmapinr-distribution.