Support `nil` as a query's input argument

[vovayartsev-dice]

Reproducible in adbc v0.7.9

This works fine

Adbc.Connection.query(conn, "SELECT $1 as name", ["Alice"])

This fails

Adbc.Connection.query(conn, "SELECT $1 as name", [nil])
# => {:error, #Adbc.Error<message: "nanoarrow call failed: PostgresType::FromSchema(type_resolver, bind_schema->children[i], &type, &na_error) = (95) Operation not supported. Can't map Arrow type 'na' to Postgres type", vendor_code: 0, ...>}

Use-case

Similar to https://arrow.apache.org/adbc/main/python/recipe/postgresql.html#loading-csv-files-into-postgresql
I actually was trying to perform INSERT INTO table (col1, col2) VALUES ($1, $2) but my dataset contains nils.

[josevalim]

A PR is welcome!

[josevalim]

It works with SQLite but not PostgreSQL, which indicates this is an issue with the postgres driver. You would have to report it upstream. :)