Fix 3810 import sizes false

[Ayudh-M]

Pull Request Checklist

• I’ve read the guidelines for contributing.
• I created a dedicated branch fix-3810-import-sizes-false.
• I updated AUTHORS.txt and CHANGES.txt.
• I ran flake8 nikola tests and pydocstyle — no new warnings.
• I built the demo site (nikola init -qd demosite && nikola build) and reproduced the WordPress import before/after the fix.

Description — fix #3810

nikola import_wordpress crashed with
TypeError: 'bool' object is not subscriptable
when a WordPress export contained "sizes" => false in _wp_attachment_metadata (WordPress ≥ 6 writes this when an image has no alternate renditions).

This patch:

• guards against non‑dict metadata[b'sizes']
• iterates only when a real dict of sizes exists
• retains existing behaviour for normal exports
• passes flake8/pydocstyle

No functional change beyond preventing the crash.

Testing

command	expected / observed
nikola import_wordpress furious-gamingde.WordPress.2025-01-12.xml --no-downloads --output new_site before patch	traceback ending with TypeError: 'bool' object is not subscriptable
same command after patch	import completes; site builds; no stack trace
same command with downloads enabled	all images imported; single‑size images processed once

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Kind request: this fix is part of a university assignment due 19 April.
The change is small (~15 lines) and isolated; any feedback would be greatly appreciated. Thank you!