[4.x] Fix: drop tenant databases on migrate fresh without domains

[MichaelKampmann]

When running php artisan migrate:fresh in a tenacy setup without domains in combination with 'drop_tenant_databases_on_migrate_fresh' => true, an ErrorException is thrown

  ErrorException 

  Attempt to read property "each" on null

  at vendor/stancl/tenancy/src/Jobs/DeleteDomains.php:24
     20▕     ) {}
     21▕ 
     22▕     public function handle(): void
     23▕     {
  ➜  24▕         $this->tenant->domains->each->delete();
     25▕     }
     26▕ }
     27▕ 

This PR resolves the issue by setting the domains to be optional on the DeleteDomain job

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.74%. Comparing base (abe1f1f) to head (7b9e65c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1282   +/-   ##
=========================================
  Coverage     84.74%   84.74%           
  Complexity     1025     1025           
=========================================
  Files           174      174           
  Lines          3015     3015           
=========================================
  Hits           2555     2555           
  Misses          460      460           

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[stancl]

Thanks!