bug: OpenImageIO fails to parse valid XMP when rdf:Description is self-closing

[StonerLing]

Hi,

I found that OpenImageIO fails to extract XMP metadata from some valid images
when the XMP uses a self-closing <rdf:Description ... /> element.

Example XMP :

<rdf:Description
    xmlns:Camera="xxxx/"
    Camera:CameraModel="xxxx"
    Camera:Pitch="-59/10"
    Camera:Roll="-27/10"
    Camera:Yaw="-459/10"
    ... />

This is valid XML, but OpenImageIO's decode_xmp() currently only looks for:

<rdf:Description> ... </rdf:Description>

and therefore does not enter the parsing loop if the tag is self-closing.

Example repro JPEG with self-closing rdf:Description XMP:

https://github.com/user-attachments/assets/30c0f3ec-322e-4f26-8450-e0371e15f1ca