fix, zustaendig_refs darf nicht `NULL` sein.

Behebt folgenden Fehler

```
F, [2014-04-02T16:20:26.418166 #6666] FATAL -- :
ActionView::Template::Error (undefined method `map' for nil:NilClass):
    47:           <td><%= link_to geschaeft.nummer, geschaeft_path(current_zone, geschaeft) %></td>
    48:           <td><%= link_to geschaeft.titel, geschaeft_path(current_zone, geschaeft) %></td>
    49:           <td class="hidden-xs"><%= geschaeft.stand %></td>
    50:           <td class="hidden-xs"><%= geschaeft.zustaendig_label %></td>
    51:           <td class="hidden-xs"><%= geschaeft.art %></td>
    52:           <td class="hidden-xs"><%= enum(:status, geschaeft.status) %></td>
    53:         </tr>
  app/models/geschaeft.rb:73:in `zustaendig'
  app/models/geschaeft.rb:69:in `zustaendig_label'
  app/views/geschaefte/_table.html.erb:50:in `block (2 levels) in _app_views_geschaefte__table_html_erb__1807157824240546812_23834416578940'
  app/views/geschaefte/_table.html.erb:45:in `block in _app_views_geschaefte__table_html_erb__1807157824240546812_23834416578940'
  app/views/geschaefte/_table.html.erb:1:in `_app_views_geschaefte__table_html_erb__1807157824240546812_23834416578940'
  app/views/verwalter/geschaefte/index.html.erb:26:in `_app_views_verwalter_geschaefte_index_html_erb___4033083274356757748_23834411052020'
```

Traceback: https://errbit.4teamwork.ch/apps/52fb8ced3838181a800040dd/problems/533c1bad3838187b29007766