[php-src] Issue #21683: pdo_pgsql crashes with ATTR_PREFETCH=0 on empty result set

Issue: https://github.com/php/php-src/issues/21683
Author: thomasschiet

### Description

The following code:

```php
<?php
$pdo = new PDO('pgsql:...');
$pdo->setAttribute(PDO::ATTR_PREFETCH, 0);

$pdo->exec("CREATE TEMP TABLE empty_t (id int, val text)");

$stmt = $pdo->query("SELECT * FROM empty_t");
var_dump($stmt->fetch());
```

Resulted in this output:
```
PHP Fatal error:  Uncaught PDOException: SQLSTATE[HY000]: General error: 7  in Command line code:9
```

But I expected this output instead:
```
bool(false)
```


### PHP Version

```plain
PHP 8.5.4 (cli) (built: Mar 10 2026 23:15:23) (NTS)
Copyright (c) The PHP Group
Built by Homebrew
Zend Engine v4.5.4, Copyright (c) Zend Technologies
    with Zend OPcache v8.5.4, Copyright (c), by Zend Technologies
```

### Operating System

_No response_