fix, GeschaeftCacher ignoriert Geschäfte ohne JournalEintrag.

Dies behebt folgende Fehler:

```
[demo@matrix geschaefte]$ bin/rake caching:refresh_geschaefte
rake aborted!
undefined method `id' for nil:NilClass
/var/www/geschaefte/app/services/geschaeft_cacher.rb:12:in `refresh'
/var/www/geschaefte/app/services/geschaeft_cacher.rb:5:in `refresh!'
/var/www/geschaefte/lib/tasks/caching.rake:6:in `block (3 levels) in <top (required)>'
/var/www/geschaefte/vendor/bundle/ruby/2.0.0/gems/activerecord-4.1.0.beta1/lib/active_record/relation/batches.rb:52:in `block (2 levels) in find_each'
/var/www/geschaefte/vendor/bundle/ruby/2.0.0/gems/activerecord-4.1.0.beta1/lib/active_record/relation/batches.rb:52:in `each'
/var/www/geschaefte/vendor/bundle/ruby/2.0.0/gems/activerecord-4.1.0.beta1/lib/active_record/relation/batches.rb:52:in `block in find_each'
/var/www/geschaefte/vendor/bundle/ruby/2.0.0/gems/activerecord-4.1.0.beta1/lib/active_record/relation/batches.rb:106:in `find_in_batches'
/var/www/geschaefte/vendor/bundle/ruby/2.0.0/gems/activerecord-4.1.0.beta1/lib/active_record/relation/batches.rb:51:in `find_each'
/var/www/geschaefte/vendor/bundle/ruby/2.0.0/gems/activerecord-4.1.0.beta1/lib/active_record/querying.rb:8:in `find_each'
/var/www/geschaefte/lib/tasks/caching.rake:5:in `block (2 levels) in <top (required)>'
Tasks: TOP => caching:refresh_geschaefte
(See full trace by running task with --trace)
```