Add default return values to method stubs

Author: rastating

lib/wpxf/wordpress/shell_upload.rb

@@ -36,14 +36,17 @@ def payload_name
 
   # @return [String] the URL of the file used to upload the payload.
   def uploader_url
+    nil
   end
 
   # @return [BodyBuilder] the {Wpxf::Utility::BodyBuilder} used to generate the uploader form.
   def payload_body_builder
+    nil
   end
 
   # @return [String] the URL of the payload after it is uploaded to the target.
   def uploaded_payload_location
+    nil
   end
 
   # Called prior to preparing and uploading the payload.