Resolve PHPStan-Strict errors: arrayFilter.strict

Problem/Motivation

Fix a bunch of errors reported by PHPStan Strict with all rules disabled, i.e, the "miscellaneous" errors before we can consider introducing strict.

Most of the things identified will be easy/sane DX things.

Steps to reproduce

Install phpstan-strict-rules, disable (=false) all rules via phpstan.neon parameters.strictRules

Remaining tasks

• CI Pass
• Review.

User interface changes

None.

Introduced terminology

None.

API changes

None.

Data model changes

None.

Release notes snippet

Nil.