api: omit missing Created field from ImageInspect response

[neersighted]

• Follow-up to Set Created to 0001-01-01T00:00:00Z on older API versions #47374

Set the Created field in image inspect to omitempty so we don't return it when it is absent from the image config, as opposed to returning an empty string.

* api: `GET /images/{id}/json` omits the `Created` field (previously it was `0001-01-01T00:00:00Z`)
  if the `Created` field is missing from the image config.

[thaJeztah]

This one probably will fail, asking for a change-log entry 😉

[tianon]

(added cherry-pick labels to follow-up #47387 👍)

[thaJeztah]

Can we still do this for v25 (as it already shipped), or is this considered a change to the API?

[thaJeztah]

Discussed in a call, and the missing omitempty was an oversight, so to not complicate things too much, it's better to include it than to have 3 variants.

[thaJeztah]

LGTM