chore: add phpstan-strict-rules and enable disallowedConstructs

[kenjis]

Description
To disallow empty() - it's a very loose comparison (see manual), it's recommended to use more strict one. See #7896 (comment)

• add https://github.com/phpstan/phpstan-strict-rules
• enable the rule disallowedConstructs

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• [] User guide updated
• Conforms to style guide