feat: fix legend rendering error for `null` category #730

Job	Run time
⬢ Prepare Node.js	31s
🐍 Prepare Python	35s
🔍 Check if pull request	2s
🧱 Build test matrix	7s
🗃 Build datasets	2m 28s
🧱 Build Spotlight	3m 55s
📝 Build API docs	43s
🔍 Check	2m 16s
🧪 Doc Test	1m 25s
🧪 Unit Test	2m 3s
🔍 Check wheel contents	33s
🧪 UI Test (chrome)	3m 35s
🧪 Integration Test (ubuntu-latest, 3.8)	11m 28s
🧪 UI Test (firefox)	3m 58s
🚀 Release Spotlight	0s
	33m 39s

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