Panic encoding empty Any

[vgough]

If a struct contains an Any value, and the Any value is left unset, then Marshal fails with a nil pointer dereference.

eg

package main

import (
    "fmt"

    jsoniter "github.com/json-iterator/go"
)

type Foo struct {
    A jsoniter.Any
}

func main() {
    data, _ := jsoniter.Marshal(&Foo{})
    fmt.Println(string(data))
}

The SEGV occurs here:

github.com/json-iterator/go.(*directAnyCodec).Encode(0x132e3a0, 0xc00000e990, 0xc000058180)
github.com/json-iterator/go@v1.1.5/any.go:315 +0x29