bug: relative asset path but no asset directory specified

[geoHeil]

https://github.com/docling-project/docling-core/blob/main/docling_core/types/doc/document.py#L4542 assumes that either:

• the path is absolute
• the path is relative and a asset directory is specified

in these cases it works

But if the artifacts_dir directory is not specified and the path is relative - then the path is prefixed with itself / duplicated

[geoHeil]

see #363

[dolfim-ibm]

Good catch, and thanks for starting to work on it.