There is one good reason why NULL should be ignored - it is used in Latte via addAttributes to clear those attributes from the element, so outputting NULL values would lead to duplicate `data-x` attribute - once with value from template and once with NULL value.
But the NULL values screw up the count of data attributes in HtmlDataset, so I guess the right thing to do is treat `$el->data->attr = NULL` as if `unset($el->data->attr)` was called.