Add missing nullability checks during data retrieval

Several data retrieval calls could potentially throw an error if the data column is NULL.
A nullability check should be added to avoid this and/or create an informative error message.

https://github.com/MadeiraData/sql-clr-ics/blob/dea8c46c0cc7591d3e7872c0a096bb497eea492a/sql_clr_ics/clr_send_ics_invite.cs#L140