ini_get() analysis should be aware of whether an option exists or not

# Feature request
`ini_get()` returns `false` only when the directive specified does not exist.

`substr()` will never receive `false` in this code because the `date.timezone` ini directive always exists.
https://phpstan.org/r/6cdcdcb9-aa0c-4848-b727-4b2cf8e225e7

The above code should not report any errors.