Use `assert_nil` instead of `assert_equal nil`

Fixes #5648

test/test_document.rb

@@ -206,7 +206,7 @@ class TestDocument < JekyllUnitTest
     should "not know the specified front matter defaults" do
       assert_equal "Example slide", @document.data["title"]
       assert_equal "slide", @document.data["layout"]
-      assert_equal nil, @document.data["nested"]
+      assert_nil @document.data["nested"]
     end
   end
 

test/test_page.rb

@@ -96,7 +96,11 @@ class TestPage < JekyllUnitTest
         attrs.each do |attr, val|
           attr_str = attr.to_s
           result = page[attr_str]
-          assert_equal val, result, "For <page[\"#{attr_str}\"]>:"
+          if val.nil?
+            assert_nil result, "For <page[\"#{attr_str}\"]>:"
+          else
+            assert_equal val, result, "For <page[\"#{attr_str}\"]>:"
+          end
         end
       end
 
@@ -220,7 +224,7 @@ class TestPage < JekyllUnitTest
 
       should "return nil permalink if no permalink exists" do
         @page = setup_page("")
-        assert_equal nil, @page.permalink
+        assert_nil @page.permalink
       end
 
       should "not be writable outside of destination" do

test/test_static_file.rb

@@ -57,7 +57,7 @@ class TestStaticFile < JekyllUnitTest
 
     should "have a destination relative directory without a collection" do
       static_file = setup_static_file("root", "dir/subdir", "file.html")
-      assert_equal nil, static_file.type
+      assert_nil static_file.type
       assert_equal "dir/subdir/file.html", static_file.url
       assert_equal "dir/subdir", static_file.destination_rel_dir
     end

test/test_theme.rb

@@ -47,11 +47,11 @@ class TestTheme < JekyllUnitTest
     end
 
     should "not allow paths outside of the theme root" do
-      assert_equal nil, @theme.send(:path_for, "../../source")
+      assert_nil @theme.send(:path_for, "../../source")
     end
 
     should "return nil for paths that don't exist" do
-      assert_equal nil, @theme.send(:path_for, "foo")
+      assert_nil @theme.send(:path_for, "foo")
     end
 
     should "return the resolved path when a symlink & resolved path exists" do