Non-exhaustive in GFromJSArr instance

[dmjio]

Getting the below:

Progress: 1/2
GHCJS/Marshal/Internal.hs:(219,5)-(224,40): Non-exhaustive patterns in case

On generic serialization with a product type that contains a list. There is a partial pattern match here that I believe might be responsible.

ghcjs-base/GHCJS/Marshal/Internal.hs

Line 220 in e9e9e36

Just (a',an) -> do

I believe a Nothing case just needs to be added, if this is correct I can submit a patch.