Sounds like a bigger general problem, but I figure it is more of an edge case. Found while working on something else. E.g. when building an array from scratch via

$builder = ConstantArrayTypeBuilder::createEmpty();
$builder->setOffsetValueType(null, new BooleanType(), true);
$builder->setOffsetValueType(null, new NullType(), true);
$builder->setOffsetValueType(null, new ConstantIntegerType(17));
$array = $builder->getArray();

I am expecting the result type to be array{0: 17|bool|null, 1?: 17|null, 2?: 17} because the last appended value is not optional any more. But it was still array{0?: 17|bool|null, 1?: 17|null, 2?: 17}. This PR fixes this. That exact example is added as test case.