replace mixed with unknown in arrow datasource #1362

Job	Run time
🔍 Check if pull request	5s
⬢ Prepare Node.js	22s
🐍 Prepare Python	21s
🧱 Build test matrix	8s
🗃 Build datasets	1m 44s
📝 Build API docs	44s
🧱 Build Spotlight	2m 18s
🔍 Check	2m 16s
🧪 Unit Test	1m 11s
🧪 Doc Test	1m 34s
🔍 Check wheel contents	26s
🧪 UI Test (chrome)	3m 1s
🧪 Integration Test (ubuntu-latest, 3.8)	9m 57s
🧪 UI Test (firefox)	2m 48s
🚀 Release Spotlight	0s
	26m 55s

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