make omitzero omit nil pointers.

encode_test.go

diff --git a/encode_test.go b/encode_test.go
index a37adf9..50bd9bb 100644
--- a/encode_test.go
+++ b/encode_test.go
@@ -76,6 +76,10 @@
 		A int "a,omitempty"
 		B int "b,omitempty"
 	}{0, 0}},
+	{"{}\n", &struct {
+		A *struct{ X int } "a,omitempty"
+		B int              "b,omitempty"
+	}{nil, 0}},
 
 	// Flow flag
 	{"a: [1, 2]\n", &struct {

goyaml.go

diff --git a/goyaml.go b/goyaml.go
index e7ba7a3..84dd1a7 100644
--- a/goyaml.go
+++ b/goyaml.go
@@ -262,6 +262,8 @@
 		return v.Len() == 0
 	case reflect.Map:
 		return v.Len() == 0
+	case reflect.Ptr:
+		return v.IsNil()
 	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
 		return v.Int() == 0
 	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr: