Only fill data with default value when there is a default in the key

Author: n-peugnet

data/test_graph_default_key_type.xml

@@ -10,6 +10,7 @@
     <key id="key2" for="node" attr.name="color" attr.type="string">
         <default>yellow</default>
     </key>
+    <key id="key3" for="node" attr.name="int-no-default" attr.type="int"></key>
     <graph edgedefault="directed" parse.nodeids="canonical" parse.edgeids="canonical" parse.order="nodesfirst">
         <node id="n0">
             <desc>test node #1</desc>

graphml/graphml.go

@@ -458,10 +458,13 @@ func attributesForData(data []*Data, target KeyForElement, gml *GraphML) (map[st
 	}
 	// fill defaults for undefined keys
 	for _, k := range keysForElement(gml.Keys, target) {
+		if k.DefaultValue == "" && k.KeyType != StringType {
+			continue
+		}
 		if _, ok := attr[k.Name]; !ok {
 			val, err := valueByType(k.DefaultValue, k.KeyType, gml.keyTypeDefault)
 			if err != nil {
-				return nil, err
+				return nil, errors.New("could not parse default value for key id: " + k.ID)
 			}
 			attr[k.Name] = val
 		}

graphml/graphml_test.go

@@ -80,6 +80,10 @@ func TestGraphML_Decode_keyTypeDefault(t *testing.T) {
 	key = gml.GetKey("integer-key", KeyForNode)
 	require.NotNil(t, key, "key expected")
 	assert.Equal(t, key.KeyType, IntType)
+
+	key = gml.GetKey("color", KeyForNode)
+	require.NotNil(t, key, "key expected")
+	assert.Equal(t, key.KeyType, StringType)
 }
 
 func TestGraphML_Decode(t *testing.T) {