ENH Methods with missing nullable return types

[lekoala]

Description

More phpdoc updates Nullable return types are tricky :)

Issues

• https://github.com/silverstripe/silverstripe-framework/issues/11173

Pull request checklist

• [x] The target branch is correct
  • See picking the right version
• [x] All commits are relevant to the purpose of the PR (e.g. no debug statements, unrelated refactoring, or arbitrary linting)
  • Small amounts of additional linting are usually okay, but if it makes it hard to concentrate on the relevant changes, ask for the unrelated changes to be reverted, and submitted as a separate PR.
• [x] The commit messages follow our commit message guidelines
• [x] The PR follows our contribution guidelines
• [x] Code changes follow our coding conventions
• [x] This change is covered with tests (or tests aren't necessary for this change)
• [x] Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
• [x] CI is green

[GuySartorelli]

As with all ENH commits, please target the next minor release branch, which is 5. You may need to reset your commits after changing the target branch. 5.1 or any other x.y branch is only for content that will be included in a patch release.

[GuySartorelli]

Nevermind, ignore that. I just realised this is jsut PHP docs so 5.1 is fine.