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

✨ Changes

Select what type of change your PR is:

• 🚀 New feature (non-breaking change which adds functionality)
• [x ] 🐞 Bug fix (non-breaking change which fixes an issue)
• 🔄 Refactor (non-breaking change which refactors the code base)
• ⚡ Performance improvements
• 🎨 Style changes (code style/formatting)
• 🧪 Tests (adding/modifying tests)
• 📚 Documentation update
• 📦 Build system changes
• 🚧 CI/CD configuration
• 🔧 Chore (general maintenance)
• 🔒 Security update
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)

✅ Checklist

Before you submit your pull request, please make sure you have completed the following steps:

• 📚 I have made the necessary updates to the documentation (if applicable).
• 🧪 I have written tests that support my changes and prove that my fix is effective or my feature works (if applicable).
• 🏷️ My PR title follows conventional commit format.

For more information about code review checklists, see the Code Review Checklist.

[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

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[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"]