[12.x] Add early return to Str::unwrap() for empty wrappers

[sumaiazaman]

This PR adds an early return optimization to the Str::unwrap() method when both wrappers are empty strings.

Changes:

• Added early return check when both $before and $after are empty
• Added check to skip startsWith() when $before is empty
• Avoids unnecessary function calls for edge cases
• Improves performance and maintains consistency with Str::start() and Str::finish()

Before:

public static function unwrap($value, $before, $after = null)
{
    if (static::startsWith($value, $before)) {
        $value = static::substr($value, static::length($before));
    }

    if (static::endsWith($value, $after ??= $before)) {
        $value = static::substr($value, 0, -static::length($after));
    }

    return $value;
}

After:

public static function unwrap($value, $before, $after = null)
{
    if ($before === '' && ($after === '' || $after === null)) {
        return $value;
    }

    if ($before !== '' && static::startsWith($value, $before)) {
        $value = static::substr($value, static::length($before));
    }

    if (static::endsWith($value, $after ??= $before)) {
        $value = static::substr($value, 0, -static::length($after));
    }

    return $value;
}