People without any Forecast assignments are silently dropped

[yongrenjie]

(via @LevanBokeria, thanks for reporting! 😊)

https://github.com/alan-turing-institute/whatwhat/blob/a2eaa5389ca52c52c57c056de90da5f9af24f8d2/lib/schedule.ml#L199-L215

These lines drop anybody who simply doesn't have any assignments at all.

They should really be reported as 100% unassigned