Summary:
In many cases, the onExport handler doesn't try to customise the format, but merely fetch some additional information before creating an export.
By allowing the `onExport` handler to also return nothing, and instead merely update existing state, this case will be easier to express now;
Reviewed By: nikoant
Differential Revision: D28026558
fbshipit-source-id: 2b90b3e1ced6a6a5b42938b6f6b74b0eb9ceafc0
d26ea5fa46