Don't fail if the returned value is nil because we might have nested …

…properties.

lib/kartograph/property.rb

@@ -25,6 +25,7 @@ def key
 
     def value_for(object, scope = nil)
       value = object.send(name)
+      return if value.nil?
       map ? artist_value(value, scope) : value
     end
 

spec/lib/kartograph/property_spec.rb

@@ -109,6 +109,18 @@
           ])
         end
       end
+
+      context 'when the value for the root object is nil' do
+        it 'returns nil' do
+          top_level = Kartograph::Property.new(:sammy) do
+            property :cephalopod
+          end
+
+          root = double(sammy: nil)
+
+          expect(top_level.value_for(root)).to be_nil
+        end
+      end
     end
   end