[5.5] Fix Problem with using old() in templates

[ameenross]

• Laravel Version: 5.5.*
• PHP Version: 7.1.7
• Database Driver & Version: N/A

Description:

According to the docs, it's possible to use the old() method in Blade templates. However, when rendering views manually (via tinker, for example), it breaks. You have to jump through hoops to get it working again. The reason is this line of code:

// @see https://github.com/laravel/framework/blob/5.5/src/Illuminate/Http/Concerns/InteractsWithFlashData.php#L16
return $this->session()->getOldInput($key, $default);

The functionality has a hard requirement on a session being available in the request. This is obviously not the case when using tinker. This PR fixes that by checking whether the session exists.

[andrewicarlson]

This is just what I needed. I was getting exceptions on all HTTP error pages (404, 500, etc) due to session not being set. Thanks!