Fix collators not producing blocks when selected candidates is zero

[tmpolaczyk]

What does it do?

Fix collators not producing blocks when selected candidates is zero.

What important points reviewers should know?

Is there something left for follow-up PRs?

What alternative implementations were considered?

Are there relevant PRs or issues in other repositories (Substrate, Polkadot, Frontier, Cumulus)?

What value does it bring to the blockchain users?

[girazoki]

I'm unsure whether I am missing something, so I would like to see a test proving this. Also adding @librelois to the review. I think the pr is correct though

The proposed test should: potentially insert manually candidates with less than minStk on storage, go to a round session change, and call this runtime API. I think this would prove that the we are doing correct things

[tmpolaczyk]

Added test for the generic case when compute_top_candidates returns an empty vec, by doing:

• Initialize candidate pool with only Alice as a collator
• Make Alice call go_offline, resulting in no candidates
• Ensure that Alice still can author blocks