convert nil to bibtex value when sorting

fix #77

features/sorting.feature

@@ -34,6 +34,42 @@ Feature: Sorting BibTeX Bibliographies
     And the "_site/scholar.html" file should exist
     Then "2007" should come before "2008" in "_site/scholar.html"
 
+  @tags @sorting
+  Scenario: Sort By Year With Nil Values
+    Given I have a scholar configuration with:
+      | key     | value             |
+      | sort_by | year              |
+    And I have a "_bibliography" directory
+    And I have a file "_bibliography/references.bib":
+      """
+      @book{ruby1,
+        title     = {The Ruby Programming Language},
+        author    = {Flanagan, David and Matsumoto, Yukihiro},
+        year      = {2008},
+        publisher = {O'Reilly Media}
+      }
+      @book{ruby2,
+        title     = {Nil}
+      }
+      @book{ruby3,
+        title     = {The Ruby Programming Language},
+        author    = {Flanagan, David and Matsumoto, Yukihiro},
+        year      = {2007},
+        publisher = {O'Reilly Media}
+      }
+      """
+    And I have a page "scholar.html":
+      """
+      ---
+      ---
+      {% bibliography -f references %}
+      """
+    When I run jekyll
+    Then the _site directory should exist
+    And the "_site/scholar.html" file should exist
+    Then "Nil" should come before "Ruby" in "_site/scholar.html"
+    And "2007" should come before "2008" in "_site/scholar.html"
+
   @tags @sorting
   Scenario: Reverse Sort Order
     Given I have a scholar configuration with:

lib/jekyll/scholar/utilities.rb

@@ -141,7 +141,10 @@ module Jekyll
       def sort(unsorted)
         return unsorted if skip_sort?
 
-        sorted = unsorted.sort_by { |e| e.values_at(*sort_keys) }
+        sorted = unsorted.sort_by do |e|
+          e.values_at(*sort_keys).map { |v| v.nil? ? BibTeX::Value.new : v }
+        end
+
         sorted.reverse! if config['order'] =~ /^(desc|reverse)/i
         sorted
       end