Prevent exceptions when accessing @appdata

For many distributions we don't load @appdata. @appdata is still
accessed, expecting that it always exists and contains :apps and
:categories. With this change, those accesses don't throw exceptions.

Fixes openSUSE#885.

app/controllers/application_controller.rb

@@ -164,6 +164,8 @@ def prepare_appdata
                  leap_appdata(@testing_version)
                when "openSUSE:Leap:#{@legacy_release}"
                  leap_appdata(@legacy_release)
+               else
+                 { apps: [], categories: Set.new }
                end
   end