Add element NotNil function (#130)

NotNil returns the receiver element if it isn't nil; otherwise, it returns
an unparented element with an empty string tag. This function simplifies
the task of writing code to ignore not-found results from element queries.

etree.go

@@ -1001,6 +1001,25 @@ func (e *Element) FindElementsPath(path Path) []*Element {
 	return p.traverse(e, path)
 }
 
+// NotNil returns the receiver element if it isn't nil; otherwise, it returns
+// an unparented element with an empty string tag. This function simplifies
+// the task of writing code to ignore not-found results from element queries.
+// For example, instead of writing this:
+//
+//	if e := doc.SelectElement("enabled"); e != nil {
+//		e.SetText("true")
+//	}
+//
+// You could write this:
+//
+//	doc.SelectElement("enabled").NotNil().SetText("true")
+func (e *Element) NotNil() *Element {
+	if e == nil {
+		return NewElement("")
+	}
+	return e
+}
+
 // GetPath returns the absolute path of the element. The absolute path is the
 // full path from the document's root.
 func (e *Element) GetPath() string {

etree_test.go

@@ -1505,3 +1505,22 @@ func TestPreserveDuplicateAttrs(t *testing.T) {
 		checkAttr(e, 0, "attr", "test2")
 	})
 }
+
+func TestNotNil(t *testing.T) {
+	s := `<enabled>true</enabled>`
+
+	doc := newDocumentFromString(t, s)
+	doc.SelectElement("enabled").NotNil().SetText("false")
+	doc.SelectElement("visible").NotNil().SetText("true")
+
+	want := `<enabled>false</enabled>`
+	got, err := doc.WriteToString()
+	if err != nil {
+		t.Fatal("etree: failed to write document to string")
+	}
+	if got != want {
+		t.Error("etree: unexpected NotNil result")
+		t.Error("wanted:\n" + want)
+		t.Error("got:\n" + got)
+	}
+}