Fix decoding into struct with new fields

[jazg]

This fixes an issue where an encoded struct cannot be decoded into a struct containing new fields. For example, if we encode the following object:

type A struct {
    Foo pack.String `json:"foo"`
}

And later introduce a new field to the struct A:

type A struct {
    Foo pack.String `json:"foo"`
    Bar pack.Bytes  `json:"bar"`
}

We will be met with the following error when decoding the original encoded value into the new object: decoding \"Bar\": unexpected value of type <nil>. To resolve this, we simply skip the decoding process if the struct field is nil.

[tok-kkk]

Approved