fix(c/driver/postgresql): Fix ingest of streams with zero arrays

[paleolimbot]

I think this didn't work because we only ever wrote COPY output to the connection after each array, so if there were no arrays, there was no header sent!

Closes #2071.

After this PR:

library(adbcdrivermanager)
#> Warning: package 'adbcdrivermanager' was built under R version 4.3.3

con <- adbc_database_init(
  adbcpostgresql::adbcpostgresql(), 
  uri = "postgresql://localhost:5432/postgres?user=postgres&password=password"
) |> 
  adbc_connection_init()

con |> 
  execute_adbc("DROP TABLE IF EXISTS no_integers")

nanoarrow::basic_array_stream(
  list(), 
  nanoarrow::na_struct(list(x = nanoarrow::na_int32()))
) |> 
  write_adbc(con, "no_integers")

con |> 
  read_adbc("SELECT * from no_integers") |> 
  tibble::as_tibble()
#> # A tibble: 0 × 1
#> # ℹ 1 variable: x <int>

^{Created on 2024-08-11 with reprex v2.1.0}

[lidavidm]

Thanks!