fix: skip image enrichment without pages

[DebadityaHait]

Issue resolved by this Pull Request:
Resolves #3867

Description

Skip image enrichment for native charts that have neither an embedded image nor page images, preventing SimplePipeline from indexing an empty page list. The chart remains in the converted document.

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

Tests:

• uv run pytest tests/test_backend_pptx.py -q
• uv run ruff format --check docling/models/base_model.py tests/test_backend_pptx.py
• uv run ruff check docling/models/base_model.py tests/test_backend_pptx.py

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

[dolfim-ibm]

I don't think this PR is solving the reported issue. PPTX still has pages but no page image.

@DebadityaHait can you please verify this?

[DebadityaHait]

Verified. PPTX produces one logical page in result.document.pages, but SimplePipeline does not populate the separate ConversionResult.pages list used by prepare_element. On the chart fixture, document_pages=1 while conversion_result_pages=0, and the chart has no embedded image. The pre-fix regression fails at conv_res.pages[0]; with this guard the conversion succeeds and retains the chart. I’m happy to make that distinction more explicit in the test if you prefer.

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✅ All modified and coverable lines are covered by tests.

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[dolfim-ibm]

lgtm