Empty expires_at on snapshots, tiny API inconsistency

[edlerd]

Required information

• Distribution: Snap
• Distribution version: 6.1

Issue description

A tiny inconsistency with responses on the snapshot API exists.

• Instance snapshots without an expiration date:
  GET 1.0/instances/:name?project=default&recursion=2 returns expires_at: "0001-01-01T00:00:00Z".
• Volume snapshots without an expiration date:
  GET 1.0/storage-pools/:pool/volumes/custom/:name/snapshots?project=default&recursion=2 returns expires_at: null.

I think both instance and volume snapshots APIs should return the same format for an empty expiration date.