fix(video-edit): skip slowmo when factor is null

[santhreal]

A slowmo effect with factor null raised TypeError in float(None).

Skip slowmo when factor is null, like non-positive factors.

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Reviewing files that changed from the base of the PR and between 7fb5692 and 9a3b9d7.

📒 Files selected for processing (3)

• chapter5/video-edit/blender_editor.py
• chapter5/video-edit/test_slowmo_null_factor.py
• chapter5/video-edit/video_editor.py

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[bojieli]

The final commit introduces two unrelated regressions. It clamps atempo to at most 1.0, so factor=0.5 no longer speeds audio to match the video, and it calls makedirs with an empty string for a bare output filename, raising FileNotFoundError before ffmpeg runs. Please revert those two drive-by changes and keep only the null-factor handling.