Use correct changeset state for diff property

Created by: eseliger

After reopening a PR, it had no diff anymore (here)[https://k8s.sgdev.org/campaigns/Q2FtcGFpZ246MTY=]

A changesets diff is meant to be null in the API, whenever a changeset is NOT open. We previously checked only the metadata of the changeset, not the events also, which lead to inconsistent null behavior, when a webhook changed the state. I also added a docstring so such bugs are less likely to happen again.

Created by: codecov[bot]

Codecov Report

Merging #9028 into master will increase coverage by <.01%. The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master    #9028      +/-   ##
==========================================
+ Coverage   41.81%   41.81%   +<.01%     
==========================================
  Files        1314     1314              
  Lines       71085    71077       -8     
  Branches     6555     6555              
==========================================
  Hits        29723    29723              
+ Misses      38646    38639       -7     
+ Partials     2716     2715       -1

Impacted Files	Coverage Δ
...erprise/internal/campaigns/resolvers/changesets.go	54.2% <0%> (+0.74%)
internal/campaigns/types.go	23.11% <33.33%> (ø)
enterprise/internal/campaigns/service.go	36.28% <50%> (+0.3%)

Created by: ryanslade

Nice. I think there are other places where we need to make this change. We should probably make State() private and use ExternalState instead.

I'm happy to take over this PR if you like?