Just swallow it if there is no content for the excerpt to parse.

lib/jekyll/excerpt.rb

@@ -106,7 +106,7 @@ module Jekyll
     # Returns excerpt String
     def extract_excerpt(post_content)
       separator     = site.config['excerpt_separator']
-      head, _, tail = post_content.partition(separator)
+      head, _, tail = post_content.to_s.partition(separator)
 
       "" << head << "\n\n" << tail.scan(/^\[[^\]]+\]:.+$/).join("\n")
     end