chore: Require breadth >= 1 in FlattenMerge

[He-Pin]

Motivation:
Check breadth >= 1 in FlattenMerge

Modification:
Add a requirement for that.

[pjfanning]

• is it really possible to end up in a position where the requirement fails?
• if so, can we add a test?
• also, what would users of older versions see in this case? an exception, a nonsensical result?

[He-Pin]

@pjfanning This is extract of a pending PR, I can't keep 100% code coverage at $Work too.

[raboof]

I think checks like this are helpful because they make it easier to read the implementation of FlattenMerge and be confident you don't need to consider the case where breadth < 1

• is it really possible to end up in a position where the requirement fails?

That's indeed not super obvious, but I suspect it can when the user chooses a <1 parallelism somewhere.

* if so, can we add a test?

I guess so, though I'm not sure it's worth it to test error checking so explicitly

* also, what would users of older versions see in this case? an exception, a nonsensical result?

I suspect they'd see a flow which stalls (fails to pull) after accepting its first element.