Rule to prefer !== null check over isset() for existing variable/key/property

[VincentLanglet]

When using isset() we're checking that

• The property/variable/... is defined
• The value is not null

When the value is always defined, it could be clearer to write $foo !== null instead.

[dereuromark]

This is somewhat a code sniffer rule already for me. But in PHPStan it would be easier or more correct to check the existence within a scope