# fix: do not read an undefined offset when a value argument is last

## Problem

When an argument that expects a value is the last token on the command line and
is preceded by an unconsumed non-prefixed token, the parser emits an
`Undefined array key` warning under PHP 8:

```php
$climate = new League\CLImate\CLImate();
$climate->arguments->add([
    'name' => ['prefix' => 'n', 'longPrefix' => 'name', 'defaultValue' => 'default-name'],
]);

$climate->arguments->parse(['script', 'somefile', '-n']);
// PHP Warning: Undefined array key 2 in Parser.php on line 270
```

On top of the warning, the argument is then set to an empty value instead of
falling back to its configured default, which is inconsistent with the behaviour
when a value argument is the only remaining token (that case correctly falls back
to the default).

## Root cause

`src/Argument/Parser.php`, `setArgumentValue()`:

```php
if (is_null($value)) {
    if (count($argv) === 0) {
        return $argv;
    }

    // If the value wasn't previously defined in "key=value"
    // format then define it from the next command argument.
    $nextArgvValue = $argv[$key + 1];
    ...
}
```

The remaining arguments array is not reindexed after previously matched arguments
are removed with `unset()`. `count($argv) === 0` is therefore not a reliable test
for "there is no following token": when an unconsumed token still sits at another
offset (for example `somefile` at offset 0), `count($argv)` is non-zero even
though offset `$key + 1` does not exist, so `$argv[$key + 1]` reads an undefined
key.

## The fix

Check that the next offset actually exists instead of relying on the overall
count. When there is no following token the method returns without setting a
value, so the argument falls back to its default (matching the existing
last-token behaviour):

```php
if (is_null($value)) {
    if (!isset($argv[$key + 1])) {
        return $argv;
    }

    $nextArgvValue = $argv[$key + 1];
    if ($this->isValidArgumentValue($nextArgvValue)) {
        $argument->setValue($nextArgvValue);
        unset($argv[$key + 1]);
        return $argv;
    }
}
```

## Test

Added `ManagerTest::testItParsesAValueTakingArgumentThatIsLastWithoutAWarning`,
which installs an error handler that turns any warning into an exception, parses
`['command', 'somefile', '-n']`, and asserts that no warning is raised and the
argument falls back to its default value.

## Gate

```
composer install && composer test
```

`composer test` runs `vendor/bin/phpunit`. Result: exit code `0` -
`OK (274 tests, 152 assertions)`.

Counterfactual: reverting `setArgumentValue()` to the `count($argv) === 0` guard
makes the new test fail with `ErrorException: Undefined array key 2` raised at
`Parser.php:270`.