Fix collection tests for latest guava-testlib update

[Marcono1234]

Purpose

Fixes build failure for #2784

Important: Targets the dependabot/... branch instead of main.

Description

With the fix in the latest guava-testlib version (see google/guava#7401) null support is now properly checked. But it expects that if a collection does not permit null, then creating it with null should fail as well, see guava-testlib's:

• MapCreationTester.testCreateWithNullValueUnsupported()
• CollectionCreationTester.testCreateWithNull_unsupported()

However, the previous implementation for the JsonArray and JsonObject test was using methods which implicitly converted null to JsonNull.

Checklist

• New code follows the Google Java Style Guide
  This is automatically checked by mvn verify, but can also be checked on its own using mvn spotless:check.
  Style violations can be fixed using mvn spotless:apply; this can be done in a separate commit to verify that it did not cause undesired changes.
• If necessary, new public API validates arguments, for example rejects null
• New public API has Javadoc
  • Javadoc uses @since $next-version$
    ($next-version$ is a special placeholder which is automatically replaced during release)
• If necessary, new unit tests have been added
  • Assertions in unit tests use Truth, see existing tests
  • No JUnit 3 features are used (such as extending class TestCase)
  • If this pull request fixes a bug, a new test was added for a situation which failed previously and is now fixed
• mvn clean verify javadoc:jar passes without errors

[Marcono1234]

For Eclipse this makes a difference: Otherwise it just shows the test suite name in the UI, and assertion stack traces are just Guava code, so there is no way to easily get to this test class.

Maven and IntelliJ automatically include the LinkedTreeMapSuiteTest class name in the console output / UI.

The other test suite classes here are affected as well, but changing their name (e.g. "JsonObject#asMap") to the qualified class name would make the names derived for nested tests by Guava quite verbose. Could alternatively wrap the test suites in an additional TestSuite with the class as name (similar to here), but not sure if worth it.

See also google/guava#7585

[eamonnmcmanus]

Thanks!