Test current behavior for handling empty leaf nodes

We're unsure if this is desired behavior for empty folders,
but making sure it's documented here in our tests.

For example:

foo/bar/baz # where baz is an empty directory

Generates:

foo:
  bar:
    baz: null

We also documented throwing YAML errors for invalid leafs

filetree/filetree.go

@@ -18,10 +18,9 @@ type Node struct {
 }
 
 func (n Node) MarshalYAML() (interface{}, error) {
-	switch len(n.Children) {
-	case 0:
+	if len(n.Children) == 0 {
 		return n.marshalLeaf()
-	default:
+	} else {
 		return n.marshalParent()
 	}
 }
@@ -41,6 +40,9 @@ func (n Node) marshalParent() (interface{}, error) {
 
 func (n Node) marshalLeaf() (interface{}, error) {
 	var content interface{}
+	if n.Info.IsDir() {
+		return content, nil
+	}
 
 	buf, err := ioutil.ReadFile(n.FullPath)
 	if err != nil {

filetree/filetree_test.go

@@ -30,16 +30,31 @@ var _ = Describe("filetree", func() {
 	})
 
 	Describe("NewTree", func() {
-		var rootFile, subDir, subDirFile string
+		var rootFile, subDir, subDirFile, emptyDir string
 
 		BeforeEach(func() {
 			rootFile = filepath.Join(tempRoot, "root_file.yml")
 			subDir = filepath.Join(tempRoot, "sub_dir")
 			subDirFile = filepath.Join(tempRoot, "sub_dir", "sub_dir_file.yml")
+			emptyDir = filepath.Join(tempRoot, "empty_dir")
 
 			Expect(ioutil.WriteFile(rootFile, []byte("foo:\n  bar"), 0600)).To(Succeed())
 			Expect(os.Mkdir(subDir, 0700)).To(Succeed())
 			Expect(ioutil.WriteFile(subDirFile, []byte("foo:\n  bar:\n    baz"), 0600)).To(Succeed())
+			Expect(os.Mkdir(emptyDir, 0700)).To(Succeed())
+
+		})
+
+		It("Throws an error if content is unmarshallable", func() {
+			anotherDir := filepath.Join(tempRoot, "another_dir")
+			anotherDirFile := filepath.Join(tempRoot, "another_dir", "another_dir_file.yml")
+			Expect(os.Mkdir(anotherDir, 0700)).To(Succeed())
+			Expect(ioutil.WriteFile(anotherDirFile, []byte("1some: in: valid: yaml"), 0600)).To(Succeed())
+			tree, err := filetree.NewTree(tempRoot)
+			Expect(err).ToNot(HaveOccurred())
+
+			_, err = yaml.Marshal(tree)
+			Expect(err).To(MatchError("yaml: mapping values are not allowed in this context"))
 		})
 
 		It("Builds a tree of the nested file-structure", func() {
@@ -49,18 +64,22 @@ var _ = Describe("filetree", func() {
 			Expect(tree.FullPath).To(Equal(tempRoot))
 			Expect(tree.Info.Name()).To(Equal(filepath.Base(tempRoot)))
 
-			Expect(tree.Children).To(HaveLen(2))
+			Expect(tree.Children).To(HaveLen(3))
 			sort.Slice(tree.Children, func(i, j int) bool {
 				return tree.Children[i].FullPath < tree.Children[j].FullPath
 			})
-			Expect(tree.Children[0].Info.Name()).To(Equal("root_file.yml"))
-			Expect(tree.Children[0].FullPath).To(Equal(rootFile))
-			Expect(tree.Children[1].Info.Name()).To(Equal("sub_dir"))
-			Expect(tree.Children[1].FullPath).To(Equal(subDir))
-
-			Expect(tree.Children[1].Children).To(HaveLen(1))
-			Expect(tree.Children[1].Children[0].Info.Name()).To(Equal("sub_dir_file.yml"))
-			Expect(tree.Children[1].Children[0].FullPath).To(Equal(subDirFile))
+
+			Expect(tree.Children[0].Info.Name()).To(Equal("empty_dir"))
+			Expect(tree.Children[0].FullPath).To(Equal(emptyDir))
+
+			Expect(tree.Children[1].Info.Name()).To(Equal("root_file.yml"))
+			Expect(tree.Children[1].FullPath).To(Equal(rootFile))
+			Expect(tree.Children[2].Info.Name()).To(Equal("sub_dir"))
+			Expect(tree.Children[2].FullPath).To(Equal(subDir))
+
+			Expect(tree.Children[2].Children).To(HaveLen(1))
+			Expect(tree.Children[2].Children[0].Info.Name()).To(Equal("sub_dir_file.yml"))
+			Expect(tree.Children[2].Children[0].FullPath).To(Equal(subDirFile))
 		})
 
 		It("renders to YAML", func() {
@@ -72,6 +91,7 @@ var _ = Describe("filetree", func() {
 			Expect(out).To(MatchYAML(`root_file.yml:
   foo:
     bar
+empty_dir: null
 sub_dir:
   sub_dir_file.yml:
     foo: