What is the use case and the expected behavior for a Flatten transform with no input?

[Shen Li]

Hi,

The FlattenTest enforces that Flatten transform can handle empty input. Is
there any use case for this feature?

https://github.com/apache/beam/blob/release-2.0.0/sdks/java/core/src/test/java/org/apache/beam/sdk/transforms/FlattenTest.java#L128

How should a runner translate an empty Flatten? Should it be a
BoundedSource that emits a single watermark of value
BoundedWindow.TIMESTAMP_MAX_VALUE?

Thanks,

Shen