[php-src] Issue #23232: is_callable('\::method') asks the autoloader for an empty class name

Issue: https://github.com/php/php-src/issues/23232
Author: spawnia

### Description

`is_callable()` on a string of the form `'\::method'` asks the autoloader for the **empty class name**. The same string without the leading backslash, `'::method'`, does not autoload at all. A leading `\` only marks the name as fully qualified, so this asymmetry looks like a bug.

The following code:

```php
<?php
spl_autoload_register(function (string $class): void {
    echo "autoload: '$class'\n";
});

foreach (['::a', '\::a', '\Foo::a', 'Foo::a'] as $callable) {
    echo "is_callable(\"$callable\")\n";
    is_callable($callable);
}
```

Resulted in this output:

```
is_callable("::a")
is_callable("\::a")
autoload: ''
is_callable("\Foo::a")
autoload: 'Foo'
is_callable("Foo::a")
autoload: 'Foo'
```

But I expected this output instead:

```
is_callable("::a")
is_callable("\::a")
is_callable("\Foo::a")
autoload: 'Foo'
is_callable("Foo::a")
autoload: 'Foo'
```

Reproduced unchanged on 8.3.32, 8.4.24 and 8.5.9 (official `php:*-cli` images).

An empty class name can never resolve, so autoloading it is pure overhead — and it is observable overhead. Composer's `ClassLoader::findFileWithExtension()` does `$first = $class[0];`, which on `''` emits `Warning: Uninitialized string offset 0`. Any application that promotes warnings to exceptions (Laravel's `HandleExceptions`, for example) then fails.

We hit this in CI: Laravel's `Factory::expandAttributes()` calls `is_callable()` on every string attribute, and a randomly generated password starting with `\::` (about 1 in a million with Faker's `password()`) made unrelated tests error out with `ErrorException: Uninitialized string offset 0`. Fixes are also filed with [composer](https://github.com/composer/composer/pull/13031) and [laravel](https://github.com/laravel/framework/pull/61151), but the autoload request for `''` looks wrong regardless of who else guards against it.

I am happy to prepare a patch rejecting the empty class name before autoload if you agree this should change.

### PHP Version

```
PHP 8.3.32, 8.4.24, 8.5.9
```

### Operating System

Linux (official `php:8.3-cli`, `php:8.4-cli`, `php:8.5-cli` images)