fix panic when fieldspec refers an empty value

kyaml/yaml/fns.go

@@ -467,6 +467,10 @@ func (s FieldSetter) Filter(rn *RNode) (*RNode, error) {
 		if err := ErrorIfInvalid(rn, yaml.ScalarNode); err != nil {
 			return rn, err
 		}
+		// value is nil or null
+		if IsMissingOrNull(s.Value) {
+			return rn, nil
+		}
 		// only apply the style if there is not an existing style
 		// or we want to override it
 		if !s.OverrideStyle || s.Value.YNode().Style == 0 {

kyaml/yaml/fns_test.go

@@ -395,6 +395,19 @@ bar: buz
 	assert.Equal(t, `baz
 `, assertNoErrorString(t)(k.String()))
 
+	// Empty value
+	node, err = Parse(`
+foo
+`)
+	assert.NoError(t, err)
+	instance = FieldSetter{}
+	k, err = instance.Filter(node)
+	assert.NoError(t, err)
+	assert.Equal(t, `foo
+`, assertNoErrorString(t)(node.String()))
+	assert.Equal(t, `foo
+`, assertNoErrorString(t)(k.String()))
+
 	// Encounter error
 	node, err = Parse(`
 -a