Apply assorted non-functional improvements

[Stephan202]

Addresses this comment and a few other things.

Suggested commit message:

Apply assorted non-functional improvements (#2054)

Summary of changes:
- Improve the intentionality of some code in `AssertJNullnessAssertion`.
- Document a possible improvement of `ExplicitArgumentEnumeration`.
- Add class documentation to several `AssertJ*Rules` classes.
- Update the Checkstyle configuration to better match the 
  Picnic-internal codebase.
- Simplify the Surefire system properties configuration.

[github-actions]

Suggested commit message:

Apply assorted non-functional improvements (#2054)

Clarify `AssertJNullnessAssertion` fix construction and note a possible
follow-up for `ExplicitArgumentEnumeration`.

Add brief class docs to the assorted `AssertJ*Rules` collections.

Align Checkstyle and Surefire settings with internal defaults by
moving system properties and enabling additional checks.

[github-actions]

Looks good. All 2 mutations in this change were killed.

class	surviving	killed
🎉tech.picnic.errorprone.bugpatterns.AssertJNullnessAssertion	0	2

Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

• Arcmutate licence valid until 2027-10-20 (yyyy/mm/dd) (non-commercial use only)
• Project uses Spring, but the Arcmutate Spring plugin is not present.
• TestNG is on the classpath but the pitest TestNG plugin is not installed.

[github-actions]

Looks good. All 2 mutations in this change were killed.

class	surviving	killed
🎉tech.picnic.errorprone.bugpatterns.AssertJNullnessAssertion	0	2

Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

• Arcmutate licence valid until 2027-10-20 (yyyy/mm/dd) (non-commercial use only)
• Project uses Spring, but the Arcmutate Spring plugin is not present.
• TestNG is on the classpath but the pitest TestNG plugin is not installed.

[rickie]

LGTM! Nice 🚀

[rickie]

Doesn't our formatter already do this?

[Stephan202]

For Java files: yes. But in the past we e.g. didn't have this for .eps files; I hope this one covers that. (But didn't check.)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Looks good. All 2 mutations in this change were killed.

class	surviving	killed
🎉tech.picnic.errorprone.bugpatterns.AssertJNullnessAssertion	0	2

Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

• Arcmutate licence valid until 2027-10-20 (yyyy/mm/dd) (non-commercial use only)
• Project uses Spring, but the Arcmutate Spring plugin is not present.
• TestNG is on the classpath but the pitest TestNG plugin is not installed.