wiki_pages/new: fix exception in rendering html data attributes.

Fix exception when trying to get the category_name for the html data
attributes. The wiki title is nil here and Tag.category_for couldn't
handle a nil tag name.

app/models/tag.rb

@@ -158,6 +158,8 @@ class Tag < ApplicationRecord
       end
 
       def category_for(tag_name, options = {})
+        return Tag.categories.general if tag_name.blank?
+
         if options[:disable_caching]
           select_category_for(tag_name)
         else