Fix defaults for structs with missing fields + Vararg fieldtypes

[quinnj]

Fixes #461. The issue here is that in the conversion from a column of a struct that had a field with type Union{T, Missing} where T was a type that had any Tuple{Vararg} fieldtypes, then the ArrowTypes.default call blew up (fieldcount not defined). To resolve, we could try to define a default for Tuple{Vararg}, but that seemed pretty messy and relying on some pretty deep reflection internals that aren't well-exposed (T.parameters[end] isa Core.TypeofVararg!), so the alternative proposed here is that to get a struct fields default value, we actually call the default on the entire struct and get the default from that. I think this should be more robust because we have pretty sensible defaults (pun intended) on most types, but when it comes to field types specifically, there are a lot of weird edge cases that are just hard to handle, like vararg fields. In this way, we don't try to mess with field type directly and instead rely on being able to get the default of the parent struct on the way down.

[quinnj]

Hmmmm, maybe this isn't actually going to work. The problem here is custom structs where people haven't needed to define default themselves, because we've always just defined default on structs as the default of their fields. kk, closing in favor of another solution.