Correctly sort the shoot based on `issue since` constraint

[acumino]

What would you like to be added:
When sorting the shoot with issue since, empty time(null) is considered the highest value and shoot with no issues comes at the top, which should not be the case.

[gardener-ci-robot]

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/lifecycle stale

[gardener-robot]

@gardener-ci-robot Command /add is not available to you but only to a Maintainer, Member, Author.

[gardener-ci-robot]

The Gardener project currently lacks enough active contributors to adequately respond to all issues.
This bot triages issues according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 60d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
• After 60d of inactivity since lifecycle/rotten was applied, the issue is closed

You can:

• Mark this issue as active with /lifecycle active
• Mark this issue as frozen with /lifecycle frozen
• Mark this issue as fresh with /remove-lifecycle stale
• Mark this issue as rotten with /lifecycle rotten
• Close this issue with /close

/lifecycle stale