Sensitive Password Exposure in Error Message

**Describe the bug**

When using `pgx.Connect` with the connection string format `"host=%s port=%s user=%s password=%s dbname=%s"`, if the `user` field is left empty, the `password` field is mistakenly interpreted as the `user`. This leads to the following error:
`failed to connect to 'user=password=**** database=***': ****: failed SASL auth: FATAL: password authentication failed for user "password=****" (SQLSTATE 28P01)`

In this case, the `password` is displayed in plain text, which constitutes an unacceptable information disclosure from a security perspective.

**To Reproduce**

Steps to reproduce the behavior:
1. Use the following connection string format: `"host=%s port=%s user=%s password=%s dbname=%s"`.
2. Leave the `user` field empty, and set a valid `password`.
3. Attempt to connect using `pgx.Connect`.

Example code:
```go
package main

import (
"context"
"log"
"os"

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

func main() {
connString := "host=localhost port=5432 user= password=secret dbname=mydb"
conn, err := pgx.Connect(context.Background(), connString)
if err != nil {
log.Fatal(err)
}
defer conn.Close(context.Background())
}
```

**Expected behavior**

The connection attempt should fail with a clear error indicating that the user field is empty, without exposing the password in the error message.

**Actual behavior**

The password field is mistakenly interpreted as the user, resulting in an error that exposes the password in plain text:
failed to connect to 'user=password=**** database=***': ****: failed SASL auth: FATAL: password authentication failed for user "password=****"

Start at the pgx.Connect entry point and reproduce the issue with an empty user and a password in the connection string. Trace how the connection string is interpreted and how the connection error is rendered; done means the attempt reports an empty user without exposing the password.

Written by the indexing model from the issue text.