feat: Make every hybrid std::shared_ptr<T> nullable

[mrousavy]

Makes every std::shared_ptr<T> nullable by default. We now have to treat every std::shared_ptr as nullable in our codebase.

We still need to think about if this change makes sense.

Before:

void test(std::shared_ptr<AnotherHostObject> hostObject) {
  hostObject->sayHello();
}

registerHybridMethod("test", &MyHostObject::test, this);

const anotherHostObject = getAnotherHostObject()
myHostObject.test(anotherHostObject) // <-- works
myHostObject.test(undefined) // <-- crashes with "jsi::HostObject cannot be casted" or something

After:

void test(std::shared_ptr<AnotherHostObject> hostObject) {
  if (hostObject == nullptr) {
    // do something if it is null, maybe throw an error, maybe default behaviour
  } else {
    hostObject->sayHello();
  }
}

registerHybridMethod("test", &MyHostObject::test, this);

const anotherHostObject = getAnotherHostObject()
myHostObject.test(anotherHostObject) // <-- works
myHostObject.test(undefined) // <-- works, uses nullable branch in code

Note

This was also possible before by simply using std::optional<std::shared_ptr<T>>, so this PR simply changes that shared ptrs are nullable by default.