Improve handling of some empty documents.

stores/yaml/store.go

@@ -124,6 +124,10 @@ func (store Store) appendYamlNodeToTreeBranch(node *yaml.Node, branch sops.TreeB
 			branch = store.appendCommentToMap(key.FootComment, branch)
 		}
 	case yaml.ScalarNode:
+		// A empty document with a document start marker without comments results in null
+		if node.ShortTag() == "!!null" {
+			return branch, nil
+		}
 		return nil, fmt.Errorf("YAML documents that are values are not supported")
 	case yaml.AliasNode:
 		branch, err = store.appendYamlNodeToTreeBranch(node.Alias, branch, false)

stores/yaml/store_test.go

@@ -139,6 +139,27 @@ func TestComment4(t *testing.T) {
 	assert.Equal(t, COMMENT_4, bytes)
 }
 
+func TestEmpty(t *testing.T) {
+	// First iteration: load and store
+	branches, err := (&Store{}).LoadPlainFile([]byte(``))
+	assert.Nil(t, err)
+	assert.Equal(t, len(branches), 0)
+	bytes, err := (&Store{}).EmitPlainFile(branches)
+	assert.Nil(t, err)
+	assert.Equal(t, ``, string(bytes))
+}
+
+func TestEmpty2(t *testing.T) {
+	// First iteration: load and store
+	branches, err := (&Store{}).LoadPlainFile([]byte(`---`))
+	assert.Nil(t, err)
+	assert.Equal(t, len(branches), 1)
+	assert.Equal(t, len(branches[0]), 0)
+	bytes, err := (&Store{}).EmitPlainFile(branches)
+	assert.Nil(t, err)
+	assert.Equal(t, ``, string(bytes))
+}
+
 func TestEmitValue(t *testing.T) {
 	// First iteration: load and store
 	bytes, err := (&Store{}).EmitValue(BRANCHES[0])