While we might never be able to have 100% of our code strict, we can at
the very least do it for all of our tests. This ensures that our tests
are using our API with the types as intended by the test author, and not
silently be cast to what our API requires.
* Clean Up Documentation for Worksheet
This PR was intended to clean up Phpstan/Scrutinizer messages regarding Worksheet. It is, for the most part, straightforward, but there is one problem which complicates things. The frequently-called public method `getParent` returns `Worksheet` or `null` but is documented to return only `Worksheet`. This can be addressed in either of two ways - change the code to match the documentation (smaller number of changes but a backwards compatibility break), or change the documentation to match the code (larger number of changes but no compatibility break). I have prepared a PR for each approach, but avoiding a compatibility break seems better, so I am pushing the latter. I can switch to the other if preferred. Most existing internal calls to `getParent` are changed to use a new method `getParentOrThrow`, which will throw an exception if parent is null. These calls would all have thrown a null pointer exception anyhow in that situation, so this should not cause any new breaks.
* Scrutinizer
One false positive, and one message leading to a minor code improvement.
Adrien Crivelli
oleibman