Fix the case where username is nil, not whodunnit

I encounter the problem where paper_trails's whodunnit isn't displayed.
It turned out that `username` method returns nil.
`try` method returns nil when there's no method, so when user doesn't have `email` method,
`@user_class.find(@version.whodunnit).try(:email)` returns nil.
Then, try this out:
`nil rescue nil || :foo`
This returns nil, surprisingly.
This means if user doesn't have email method, username is always nil.
So my fix is like below:
`(nil rescue nil) || :foo`
This returns `:foo` as expected.

lib/rails_admin/extensions/paper_trail/auditing_adapter.rb

@@ -23,7 +23,7 @@ def table
         end
 
         def username
-          @user_class.find(@version.whodunnit).try(:email) rescue nil || @version.whodunnit
+          (@user_class.find(@version.whodunnit).try(:email) rescue nil) || @version.whodunnit
         end
 
         def item