Fix for custom tag rendering

lib/liquid/tag.rb

@@ -54,7 +54,8 @@ module Liquid
     # of the `render_to_output_buffer` method will become the default and the `render`
     # method will be removed.
     def render_to_output_buffer(context, output)
-      output << render(context)
+      render_result = render(context)
+      output << render_result if render_result
       output
     end