Fix doubtless HLint issues in Federator

[supersven]

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• If HTTP endpoint paths have been added or renamed, the endpoint / config-flag checklist (see Wire-employee only backend wiki page) has been followed.
• If a cassandra schema migration has been added, I ran make git-add-cassandra-schema to update the cassandra schema documentation.
• changelog.d contains the following bits of information (details):
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.
  • If new config options introduced: added usage description under docs/reference/config-options.md
  • If new config options introduced: recommended measures to be taken by on-premise instance operators.
  • If a cassandra schema migration is backwards incompatible (see also these docs), measures to be taken by on-premise instance operators are explained.
  • If a data migration (not schema migration) introduced: measures to be taken by on-premise instance operators.
  • If public end-points have been changed or added: does nginz need un upgrade?
  • If internal end-points have been added or changed: which services have to be deployed in a specific order?

[fisx]

I like this change, but I doubt that it's non-controversial. :) Let's see if anybody complains.

[supersven]

They seem to be exactly the opposite of each other:

when p s = if p then s else pure ()

unless p s =  if p then pure () else s

So, I'm wondering how this could be controversial. 🤔

[fisx]

it's clearly still correct, just a question of style: is code easier to read if with fewer keywords, or with less complexity in boolean expressions? i'm also leaning towards unless, and i think we're way too deep into a bike shedding thing again here, so just ignore me. if somebody feels strongly, they can make a PR with an hlint rule that rolls this back, then we can bike shed. :)

[fisx]

(please add a changelog line.)

[fisx]

good to merge!