Don't marshal empty byte or uint8 slice as null

[]byte or []uint8 are encoded as base-64 encoded string. Per this, non-nil
empty slice should not get marshalled as null, rather as "".

This restores compatibility with the standard library.

reflect_native.go

@@ -432,11 +432,11 @@ func (codec *base64Codec) Decode(ptr unsafe.Pointer, iter *Iterator) {
 }
 
 func (codec *base64Codec) Encode(ptr unsafe.Pointer, stream *Stream) {
-	src := *((*[]byte)(ptr))
-	if len(src) == 0 {
+	if codec.sliceType.UnsafeIsNil(ptr) {
 		stream.WriteNil()
 		return
 	}
+	src := *((*[]byte)(ptr))
 	encoding := base64.StdEncoding
 	stream.writeByte('"')
 	size := encoding.EncodedLen(len(src))