[WikiPages] Fix exception on show_or_new without arg

This now uses `titled` which didn't handle nil

app/models/wiki_page.rb

@@ -50,7 +50,7 @@ class WikiPage < ApplicationRecord
 
   module SearchMethods
     def titled(title)
-      find_by(title: title.downcase.tr(" ", "_"))
+      find_by(title: title&.downcase&.tr(" ", "_"))
     end
 
     def active

test/unit/wiki_page_test.rb

@@ -61,6 +61,7 @@ class WikiPageTest < ActiveSupport::TestCase
 
       should "search by title" do
         assert_equal("hot_potato", WikiPage.titled("hot potato").title)
+        assert_nil(WikiPage.titled(nil))
       end
 
       should "search other names with wildcards" do