Investigate why DoclingDocument.load_from_doctags requires image for bbox extraction

There are references in the code such as this: https://github.com/docling-project/docling-core/blob/main/docling_core/types/doc/document.py#L5779

Where the `extract_bounding_box` method is called only if `image is not None`, which appears to make no sense, since it works without that constraint in other call sites.

E.g. when parsing doctags that include list items, the list_item provenance is not populated.