ical-generator: start has to be a valid date

[brettski]

Seeming multiple crashes over time in the ical-generator function where start has to be a valid date.

I am assuming this is happening when sessions are active without start dates. More investigation is required.
Initial fix thoughts is to skip adding the session if doesn't have a start date, as how can you add something to a calendar when there isn't a date associated with that thing?

https://console.cloud.google.com/errors/detail/COL5y_Dnide_JQ;time=P30D?project=all-that