Evaluation page: can't parse `?` into unknown when input column type is single

[LittleLittleCloud]

All columns are single type.
It's hard to say whether it's a bug or should be expected, but it's definitely blocking try-it-out story

[zewditu]

I think this is not a bug, but we should communicate exactly what happened there

[JakeRadMSFT]

We'll revisit our error story. #1378