Postgres column doesn't roundtrip for type `Maybe (Vector Text)`

[roberth]

I have a table with

    labels :: Columnar f (Maybe (Vector Text))

Inserts work fine. Nothing and Just [] are distinct in the database, as expected. However, queries return Just [] when they should Nothing.

This might be a problem with the FromBackendRow instance for Maybe and/or Vector.

Found on beam version 6b3e4c91ebabafc79878a35648d7f78d378ce445