Allow DisplayNameGenerator to return null to fall back to the default one

[d-william]

Overview

Resolves https://github.com/junit-team/junit5/issues/2969

---

I hereby agree to the terms of the JUnit Contributor License Agreement.

---

Definition of Done

• [x] There are no TODOs left in the code
• [ ] Method preconditions are checked and documented in the method's Javadoc
• [x] Coding conventions (e.g. for logging) have been followed
• [ ] Change is covered by automated tests including corner cases, errors, and exception handling
• [x] Public API has Javadoc and @API annotations
• [ ] Change is documented in the User Guide and Release Notes
• [x] All continuous integration builds pass

[d-william]

Should I add a release note ? In which file ?

[sbrannen]

Should I add a release note ? In which file ?

Since it looks like this will not get included until 5.10 M1, the file would be release-notes-5.10.0-M1.adoc in this directory, but that file does not exist yet.

So don't worry about the release note at the moment.

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. Given the limited bandwidth of the team, it will be closed if no further activity occurs. If you intend to work on this pull request, please reopen the PR. Thank you for your contributions.

[juliette-derancourt]

This has been merged and slightly edited in 4ee85e2 and 4836c4d. Thanks for your PR @d-william!