encoding/json: marshal should not encode zero time

[eaigner]

Time.MarshalJSON also encodes uninitialized time.Time values.

It would be better if the encoding was guarded with IsZero() to avoid returning
meaningless times.

[eaigner]

Comment 1:

... in conjunction with `omitempty`

[eaigner]

Comment 2:

I don't know if this is even possible in the scope of the Time type, but I guess not.
Maybe another interface like MarshalJSON() would be required to implement this.
    type OmitEmpty interface {
      IsEmptyJSON() bool
    }

[rsc]

Comment 3:

Status changed to Duplicate.

Merged into issue #4357.