Fix references for ResultCollection.extra and improve str and repr

[carmocca]

What does this PR do?

Fixes an issue where the ResultCollection.extra return dict was not set back to the object in the default case.

Also improve its string representations.

Found while debugging #8613

Does your PR introduce any breaking changes? If yes, please list them.

None

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

[codecov]

Codecov Report

Merging #8622 (e82fbcd) into master (c99e2fe) will increase coverage by 0%.
The diff coverage is 90%.

@@          Coverage Diff           @@
##           master   #8622   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         168     168           
  Lines       13951   13956    +5     
======================================
+ Hits        12909   12914    +5     
  Misses       1042    1042