part: Fix MarshalYAML on empty maps

MarshalYAML nil deref'd the 'm.tree' when the map was empty.
Fix this and extend test coverage.

Signed-off-by: Jussi Maki <jussi.maki@isovalent.com>

part/map.go

@@ -243,9 +243,11 @@ func (m *Map[K, V]) UnmarshalJSON(data []byte) error {
 
 func (m Map[K, V]) MarshalYAML() (any, error) {
 	kvs := make([]mapKVPair[K, V], 0, m.Len())
-	iter := m.tree.Iterator()
-	for _, kv, ok := iter.Next(); ok; _, kv, ok = iter.Next() {
-		kvs = append(kvs, kv)
+	if m.tree != nil {
+		iter := m.tree.Iterator()
+		for _, kv, ok := iter.Next(); ok; _, kv, ok = iter.Next() {
+			kvs = append(kvs, kv)
+		}
 	}
 	return kvs, nil
 }

part/map_test.go

@@ -176,10 +176,16 @@ func TestMapJSON(t *testing.T) {
 
 func TestMapYAMLStringKey(t *testing.T) {
 	var m part.Map[string, int]
-	m = m.Set("foo", 1).Set("bar", 2).Set("baz", 3)
 
 	bs, err := yaml.Marshal(m)
 	require.NoError(t, err, "Marshal")
+	require.Equal(t, "[]\n", string(bs))
+
+	m = m.Set("foo", 1).Set("bar", 2).Set("baz", 3)
+
+	bs, err = yaml.Marshal(m)
+	require.NoError(t, err, "Marshal")
+	require.Equal(t, "- k: bar\n  v: 2\n- k: baz\n  v: 3\n- k: foo\n  v: 1\n", string(bs))
 
 	var m2 part.Map[string, int]
 	err = yaml.Unmarshal(bs, &m2)

part/set_test.go

@@ -75,9 +75,16 @@ func TestSetYAML(t *testing.T) {
 
 	bs, err := yaml.Marshal(s)
 	require.NoError(t, err, "Marshal")
+	require.Equal(t, "- bar\n- baz\n- foo\n", string(bs))
 
 	var s2 part.Set[string]
 	err = yaml.Unmarshal(bs, &s2)
 	require.NoError(t, err, "Unmarshal")
 	require.True(t, s.Equal(s2), "Equal")
+
+	var empty part.Set[string]
+	bs, err = yaml.Marshal(empty)
+	require.NoError(t, err, "Unmarshal")
+	require.Equal(t, "[]\n", string(bs))
+	require.True(t, s.Equal(s2), "Equal")
 }