Added a couple of null checks to v3 model

Asana spec was blowing up, now it's working as expected.

datamodel/high/3.0/document_test.go

@@ -350,6 +350,18 @@ func TestStripeAsDoc(t *testing.T) {
 	fmt.Println(d)
 }
 
+func TestAsanaAsDoc(t *testing.T) {
+	data, _ := ioutil.ReadFile("../../../test_specs/asana.yaml")
+	info, _ := datamodel.ExtractSpecInfo(data)
+	var err []error
+	doc, err = lowv3.CreateDocument(info)
+	if err != nil {
+		panic("broken something")
+	}
+	d := NewDocument(doc)
+	fmt.Println(d)
+}
+
 func TestCircularReferencesDoc(t *testing.T) {
 	data, _ := ioutil.ReadFile("../../../test_specs/circular-tests.yaml")
 	info, _ := datamodel.ExtractSpecInfo(data)

datamodel/high/3.0/external_doc.go

@@ -18,8 +18,12 @@ type ExternalDoc struct {
 func NewExternalDoc(extDoc *low.ExternalDoc) *ExternalDoc {
 	d := new(ExternalDoc)
 	d.low = extDoc
+	if !extDoc.Description.IsEmpty() {
 		d.Description = extDoc.Description.Value
+	}
+	if !extDoc.URL.IsEmpty() {
 		d.URL = extDoc.URL.Value
+	}
 	d.Extensions = high.ExtractExtensions(extDoc.Extensions)
 	return d
 }

datamodel/high/3.0/tag.go

@@ -19,9 +19,15 @@ type Tag struct {
 func NewTag(tag *low.Tag) *Tag {
 	t := new(Tag)
 	t.low = tag
+	if !tag.Name.IsEmpty() {
 		t.Name = tag.Name.Value
+	}
+	if !tag.Description.IsEmpty() {
 		t.Description = tag.Description.Value
+	}
+	if !tag.ExternalDocs.IsEmpty() {
 		t.ExternalDocs = NewExternalDoc(tag.ExternalDocs.Value)
+	}
 	t.Extensions = high.ExtractExtensions(tag.Extensions)
 	return t
 }