get procedure for default excerpt separator for both cases site and page was

moved to the post's specific method :excerpt_separator.

lib/jekyll/excerpt.rb

@@ -105,10 +105,7 @@ module Jekyll
     #
     # Returns excerpt String
     def extract_excerpt(post_content)
-      separator     = !post.excerpt_separator.empty? &&
+      head, _, tail = post_content.to_s.partition(post.excerpt_separator)
-                       post.excerpt_separator ||
-                       site.config['excerpt_separator']
-      head, _, tail = post_content.to_s.partition(separator)
 
       "" << head << "\n\n" << tail.scan(/^\[[^\]]+\]:.+$/).join("\n")
     end

lib/jekyll/post.rb

@@ -119,11 +119,12 @@ module Jekyll
       data.fetch('title') { titleized_slug }
     end
 
-    # Public: the Post excerpt_separator, from the YAML Front-Matter or empty string
+    # Public: the Post excerpt_separator, from the YAML Front-Matter or site default
+    #         excerpt_separator value
     #
     # Returns the post excerpt_separator
     def excerpt_separator
-      data.fetch('excerpt_separator') { "" }
+      data.fetch('excerpt_separator') { site.config['excerpt_separator'].to_s }
     end
 
     # Turns the post slug into a suitable title
@@ -315,7 +316,7 @@ module Jekyll
     end
 
     def generate_excerpt?
-      !site.config['excerpt_separator'].to_s.empty? || !excerpt_separator.empty?
+      !excerpt_separator.empty?
     end
   end
 end