Can't get categories of top level (parentId === 0)

# Issue report
To get tree (child categories) need to use method \AdobeStock\Api\Client\SearchCategory::getCategoryTree
This method has next lines:
`
if ($request->getCategoryId() == null) {
throw StockApiException::withMessage('Category Id cannot be null');
}
`

But categories of top level have parentId == 0. And exception thrown in this case

It could be fixed if use strict compare with null
`
if ($request->getCategoryId() === null) {
throw StockApiException::withMessage('Category Id cannot be null');
}
`