Upgrading Multisite to 3.1 with a site which hasnt a tree cause an error

[GertTimmerman]

Bug Description

I ran php please updates:run 3.0 but i encountered an error:

  ErrorException 

  Undefined index: tree

  at vendor/statamic/cms/src/UpdateScripts/UseDedicatedTrees.php:75
     71▕             }
     72▕ 
     73▕             $yaml = YAML::file($path)->parse();
     74▕ 
  ➜  75▕             return $nav->makeTree($site->handle(), $yaml['tree']);
     76▕         })->filter();
     77▕     }
     78▕ 
     79▕     private function singleSiteNavTrees($nav)

      +1 vendor frames 
  2   [internal]:0
      Statamic\UpdateScripts\UseDedicatedTrees::Statamic\UpdateScripts\{closure}(Object(Statamic\Sites\Site), "german")

      +26 vendor frames 
  29  please:37
      Illuminate\Foundation\Console\Kernel::handle(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))

How to Reproduce

I have an collection where some sites dont have any translations, they just use all the content from the origin. Some sites have translation but some don't. In this case, the german site dont have a tree, others sites do have a tree

Environment

Statamic 3.1.0 Pro
Laravel 8.34.0
PHP 7.4.15
gerttimmerman/statamic-alignment-fieldtype dev-master
joetannenbaum/antlers-stack-tags 0.1.0
optimoapps/statamic-bard-text-align V1.0.1
spatie/statamic-algolia-places 1.1.0
spatie/statamic-responsive-images 2.5.2
withcandour/aardvark-seo 2.0.10

[jasonvarga]

Did you have a nav with no tree in it?