[meet] Allow meeting without a facilitator

[domdomegg]

Currently the meet API will return an error if asked about a cohort session with no facilitator in Airtable. This seems arbitrary and it should be possible to have facilitator-less sessions (even if we don't need this in future, it makes the system more tolerant of missing data in Airtable).

This probably looks like changing the definitions in db.ts to allow an empty cell e.g. string | null rather than string for facilitator.

Maybe also it should accept no participants e.g. just facilitators.