Fix PHPStan

[fredden]

Description

When I run the test-suite locally, I get errors unless I apply the proposed fixes. Curiously these errors don't seem to be showing up in GitHub - eg https://github.com/PHPCSStandards/PHP_CodeSniffer/actions/runs/7141889620/job/19450040728.

Suggested changelog entry

A changelog entry should not be necessary for this change.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the Contribution Guidelines.
• I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
• I have verified that the code complies with the projects coding standards.

[jrfnl]

@fredden Happy to accept the PHPStan config change, not the PHPUnit change though.

[jrfnl]

Thanks @fredden !