Fix/issue 3171 dynamic properties

[rajyan]

fixes phpstan/phpstan#3171 completly

The error was suppressed by #1234
but still happening if checkDynamicProperties is enabled.
https://phpstan.org/r/7092e4dc-cfed-4fcf-810a-7ad5e3b9555e

It is because, ensureNonNullability was not traversing on ArrayDimFetch. d450536.
Also, I think the logic for ensureNonNullability and allowedUndefinedExpression should be consistent with each other, so refactored ensureNonNullability by using lookForExpressionCallback.

As I mentioned in a624060
there is one more possible false positive, property fetch on Class|false (or something like false that doesn't accept fetching properties), but I think it is too complicate to solve now.

When property fetch is called inside isset, the isset truthy scope is more specific than non-null, it can say "the fetched var has a !hasProperty->no() type". Similar type specification is possible in ArrayDimFetch too. (adding this level of specification can solve issues like phpstan/phpstan#7073, but maybe too much)

[rajyan]

The $findMethods option is deleted. b8a6d7f
it had this
https://phpstan.org/r/397d02ef-b073-4dcc-9a96-f6ad0a83dad4
difference between coalesce and isset before, but it should report error on isset too. $this->property can be null and method call on null is an fatal error even inside isset.
coalesce
https://3v4l.org/QhW5u
isset (I added an array dim fetch in this example because without that, it's another fatal error)
https://3v4l.org/a9ITP

[ondrejmirtes]

Perfect, thank you!