fix(visualization): remove fields overiddes when set to None

[donnadamus]

📝 Description

I was trying to just visualize image+heatmap, but the current code overrides the fields and if set to None, it would set it ["image", "gt_mask"], forcing the user to also save the original image

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[Copilot]

Pull Request Overview

This PR fixes a bug where the fields parameter in the Visualizer class was being overridden with a default value ["image", "gt_mask"] when set to None, preventing users from visualizing only image+heatmap combinations without including the original image field.

Key changes:

• Removed the default fallback value for the fields parameter when it is explicitly set to None

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[Copilot]

Setting self.fields = fields without a fallback allows None to be assigned directly. This could cause issues downstream if code expects self.fields to be a list. Consider checking whether fields being None is handled properly throughout the class, or document this behavior explicitly if None is a valid value for visualization logic.

Suggested change

	self.fields = fields
	self.fields = fields or ["image", "gt_mask"]

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[ashwinvaidya17]

Apologies for the super late review. Maybe it might be an idea to replace with

Suggested change

	fields: list[str] | None = ["image", "gt_mask"],

This way the default behaviour remains the same