Fix pdf_add_stream metadata error

[clpo13]

I ran into a problem with the recode_pdf tool from archive-pdf-tools, which uses PyMuPDF. This error occurs with versions of PyMuPDF from 1.23.9 and on:

$ recode_pdf --from-imagestack test001.tif --hocr-file test001.hocr --dpi 400 -o test001.pdf
Traceback (most recent call last):
  File "/home/cody/ocrtest/venv/bin/recode_pdf", line 301, in <module>
    res = recode(args.from_pdf, args.from_imagestack, args.dpi, args.hocr_file,
  File "/home/cody/ocrtest/venv/lib/python3.10/site-packages/internetarchivepdf/recode.py", line 749, in recode
    write_metadata(in_pdf, outdoc, extra_metadata=extra_metadata)
  File "/home/cody/ocrtest/venv/lib/python3.10/site-packages/internetarchivepdf/pdfhacks.py", line 516, in write_metadata
    to_pdf.set_xml_metadata(stream)
  File "/home/cody/ocrtest/venv/lib/python3.10/site-packages/fitz/__init__.py", line 5580, in set_xml_metadata
    xml = mupdf.pdf_add_stream( pdf, res, None, 0)
  File "/home/cody/ocrtest/venv/lib/python3.10/site-packages/fitz/mupdf.py", line 45040, in pdf_add_stream
    return _mupdf.pdf_add_stream(doc, buf, obj, compressed)
ValueError: invalid null reference in method 'pdf_add_stream', argument 3 of type 'mupdf::PdfObj const &'

The change in this PR follows the pattern of the other calls to mupdf.pdf_add_stream in src/__init__.py, but if there's a better way to do it, let me know.

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[clpo13]

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[clpo13]

recheck

[julian-smith-artifex-com]

This is great, many thanks for the fix, will merge.

[I have a new test in my tree that fails without your fix, which i'll push later].