Change falling back to nil instead of empty list...

Nokogiri for MRI handles [] and nil the same when calling #canonicalize
However, Nokogir for JRuby blows up with []. The default argument according
to the method definition is nil, so I believe it's more appropriate:

http://www.rubydoc.info/github/sparklemotion/nokogiri/Nokogiri/XML/Node#canonicalize-instance_method

Author: ylansegal

lib/xml_security.rb

@@ -336,7 +336,7 @@ def extract_inclusive_namespaces
         prefix_list = element.attributes.get_attribute("PrefixList").value
         prefix_list.split(" ")
       else
-        []
+        nil
       end
     end
 

test/xml_security_test.rb

@@ -168,14 +168,14 @@ class XmlSecurityTest < Minitest::Test
         assert response.validate!
       end
 
-      it "return an empty list when inclusive namespace element is missing" do
+      it "return nil when inclusive namespace element is missing" do
         response = fixture(:no_signature_ns, false)
         response.slice! %r{<InclusiveNamespaces xmlns="http://www.w3.org/2001/10/xml-exc-c14n#" PrefixList="#default saml ds xs xsi"/>}
 
         document = XMLSecurity::SignedDocument.new(response)
         inclusive_namespaces = document.send(:extract_inclusive_namespaces)
 
-        assert inclusive_namespaces.empty?
+        assert inclusive_namespaces.nil?
       end
     end