fix crash when opening configuration page

[deischi]

I don't speak Ruby, so please check if this is the right way handle this error message:

ActionView::Template::Error (undefined method `[]' for nil:NilClass):
3:

<%= l(:issue_status_change_rake_task) %>

4:
5:
6: <% settings['status_change']['close'] = Hash.new if settings['status_change']['close'] == nil %>
7: <% settings['new_status']['close'] = Hash.new if settings['new_status']['close'] == nil %>
8:
9: <% settings['status_change']['open'] = Hash.new if settings['status_change']['open'] == nil %>

plugins/redmine_issue_status_changer/app/views/settings/_issue_status_change.html.erb:6:in _plugins_redmine_issue_status_changer_app_views_settings__issue_status_change_html_erb___2976537341796933126_88720' app/views/settings/plugin.html.erb:6:in block in _app_views_settings_plugin_html_erb___293412427546100677_88700'
app/views/settings/plugin.html.erb:4:in `_app_views_settings_plugin_html_erb___293412427546100677_88700'