✨ Add minimum node height & 0 value fallback color

[joao-m-santos]

Closes #328

📝 Description

• Added a minimum height of 2px to the alluvial nodes
• Added a -1px Y offset for nodes that have this 2px min. height
• Fallback color is now grey if the node value is 0

💥 Is this a breaking change (Yes/No):

• [x] No
• [ ] Yes (please describe the impact and migration path for existing Lume users)

📝 Additional Information

Before submitting the PR, please make sure you do the following

• [x] Read the Contributing Guidelines.
• [x] Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• [x] Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• [x] Ideally, include relevant tests that fail without this PR but pass with it.

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