handle edge cases; group has no organizer, old events

[EconomiCurtis]

For example, here's a group with no organizer;

r_user_group = find_groups("Durban r-user")
#> Downloading 1 record(s)...
#> Error: Result 1 is not a length 1 atomic vector

I replaced .null with .default because purrr issue "Default value to replace NULL #110"

[EconomiCurtis]

Adding more default values also helps deal with issue #29.