fix: properly handle missing page image case for export_to_html

[kmyusk]

Fixes #165

[mergify]

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🟢 Enforce conventional commit

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Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/

• title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?(!)?:

🟢 Require two reviewer for test updates

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[kmyusk]

CI is failing on a file I did not touch FAILED test/test_docling_doc.py::test_formula_mathml - assert '<!DOCTYPE ht...pre>\n</html>' == '<!DOCTYPE ht...div>\n</html>'

[PeterStaar-IBM]

Nice!