`EXPLAIN (GENERIC_PLAN)` does not correctly handle 0 args

**Describe the bug**

When using `EXPLAIN (GENERIC_PLAN)` on a [PG16 database](https://www.postgresql.org/docs/16/sql-explain.html), pgx incorrectly tries to assign values from the args to `Query` to the query parameters. In this scenario it should probably error if _any_ args are provided instead.

**To Reproduce**

- Given the database schema (in a postgres 16 DB)
```sql
create table items (
item_id integer not null,
name text not null,

primary key (item_id)
);
```

- And the following go code:

```go
// main.go
package main

import (
"context"
"log"
"os"
"os/signal"

"github.com/jackc/pgx/v5"
)

func main() {
ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt)
defer cancel()

conn, err := pgx.Connect(ctx, os.Getenv("DATABASE_URL"))
if err != nil {
log.Fatal("opening db connection: ", err)
}
defer func() { _ = conn.Close(ctx) }()

var plan string
if err := conn.QueryRow(
ctx,
`explain (generic_plan) select items.name from items where items.item_id = $1`,
// ! passes no args, as we want to understand the generic query plan
).Scan(&plan); err != nil {
log.Fatal("executing query: ", err)
}

fmt.Println(plan)
}
```

> I'll note this also applies to the latest patch of `pgx/v4`, where I first found this

- Running

```shell
go run -race ./main.go
```

- Gives

```
2024/09/27 19:33:00 executing query: expected 1 arguments, got 0
exit status 1
```

**Expected behavior**

I expected pgx to recognise that `explain (generic_plan)` does not require arguments to be provided, accept the query, and return the query plan provided by the database

PSQL correctly returns the query plan as expected

```
$ psql --no-psqlrc --tuples-only -c 'explain (generic_plan) select items.name from items where items.item_id = $1'
Index Scan using items_pkey on items (cost=0.15..8.17 rows=1 width=32)
Index Cond: (item_id = $1)
```

**Actual behavior**

pgx returns an error, as no arguments were provided to the query.

```
expected 1 arguments, got 0
```

**Version**

- Go: `go version go1.23.1 darwin/arm64`
- PostgreSQL: `PostgreSQL 16.3 (Debian 16.3-1.pgdg120+1) on aarch64-unknown-linux-gnu, compiled by gcc (Debian 12.2.0-14) 12.2.0, 64-bit` (a freshly installed docker container running on an M1 Pro)
- pgx: `github.com/jackc/pgx/v5 v5.7.1`
- also present in `github.com/jackc/pgx/v4 v4.18.3`

**Additional context**

- [pq](https://github.com/lib/pq) correctly handles this behaviour, but I suspect this is more by luck than design as it [short-circuits to `simpleQuery`](https://github.com/lib/pq/blob/master/conn.go#L901-L905) if no args are provided

Start at the conn.QueryRow entry point and reproduce the reported behavior against PostgreSQL 16 using the provided Go example. Trace how EXPLAIN (GENERIC_PLAN) handles its placeholders when no arguments are supplied; done means the query returns the generic plan as expected and argument handling remains correct.

Written by the indexing model from the issue text.