Delete all null entries before removing the column

related to #7032

Signed-off-by: Lukas Matt <lukas@zauberstuhl.de>

closes #7038

db/migrate/20160225232049_link_share_visibilities_with_user.rb

@@ -24,11 +24,12 @@ class LinkShareVisibilitiesWithUser < ActiveRecord::Migration
     remove_index :share_visibilities, name: :shareable_and_hidden_and_contact_id
 
     remove_column :share_visibilities, :contact_id
-    change_column :share_visibilities, :user_id, :integer, null: false
 
     ShareVisibility.joins("LEFT OUTER JOIN users ON users.id = share_visibilities.user_id")
       .delete_all("users.id is NULL")
 
+    change_column :share_visibilities, :user_id, :integer, null: false
+
     add_index :share_visibilities, :user_id
     add_index :share_visibilities, %i(shareable_id shareable_type user_id), name: :shareable_and_user_id
     add_index :share_visibilities, %i(shareable_id shareable_type hidden user_id),