Steps to reproduce:

• use Markdown Wiki formatting
• Create a Wiki page
• Add an empty image tag, i.e.:

![]()

• Save the changes
• From now on, this page will show an internal server error, eventually also crashing the whole Redmine instance

The log shows the following error:

Started GET "/projects/fpga/wiki" for 127.0.0.1 at 2016-07-05 11:28:01 +0200
Processing by WikiController#show as HTML
  Parameters: {"project_id"=>"fpga"}
  Current user: viktor@foobar.com (id=17)
  Rendered wiki/_content.html.erb (9.7ms)
  Rendered wiki/show.html.erb within layouts/base (31.2ms)
Completed 500 Internal Server Error in 277ms (ActiveRecord: 23.1ms)

ActionView::Template::Error (undefined method `include?' for nil:NilClass):
    1: 

    2:   <%= textilizable content, :text, :attachments => content.page.attachments,
    3:         :edit_section_links => (@sections_editable && {:controller => 'wiki', :action => 'edit', :project_id => @page.project, :id => @page.title}) %>
    4: 
  lib/redmine/helpers/url.rb:26:in `uri_with_safe_scheme?'
  lib/redmine/wiki_formatting/markdown/formatter.rb:48:in `image'
  lib/redmine/wiki_formatting/markdown/formatter.rb:60:in `render'
  lib/redmine/wiki_formatting/markdown/formatter.rb:60:in `to_html'
  lib/redmine/wiki_formatting.rb:89:in `to_html'
  app/helpers/application_helper.rb:598:in `textilizable'
  app/views/wiki/_content.html.erb:2:in `_app_views_wiki__content_html_erb__2292234759012989548_70948640'
  app/views/wiki/show.html.erb:47:in `_app_views_wiki_show_html_erb___1795174084185801307_65213660'
  app/controllers/wiki_controller.rb:114:in `show'
  lib/redmine/sudo_mode.rb:63:in `sudo_mode'