Do not raise exception when content_type is a empty string

When content type header is blank we were raising an exception because
`empty?` was being called on nil.

actionpack/lib/action_dispatch/http/response.rb

@@ -408,7 +408,7 @@ def cookies
     def parse_content_type(content_type)
       if content_type
         type, charset = content_type.split(/;\s*charset=/)
-        type = nil if type.empty?
+        type = nil if type && type.empty?
         ContentTypeHeader.new(type, charset)
       else
         NullContentTypeHeader

actionpack/test/dispatch/response_test.rb

@@ -110,6 +110,11 @@ def test_setting_content_type_header_impacts_content_type_method
     assert_equal "application/aaron", @response.content_type
   end
 
+  def test_empty_content_type_returns_nil
+    @response.headers['Content-Type'] = ""
+    assert_equal nil, @response.content_type
+  end
+
   test "simple output" do
     @response.body = "Hello, World!"