type check allows nil

[Bastrabun]

https://github.com/mt-mods/mail/blob/f7cc435f6ba2b7aa82940b112880ad16e7efe0f3/api.lua#L25C1-L25C79

What this means is "Under what circumstances would this expression evaluate to no, that's not a string?:"

	if type(m.to or "") ~= "string" then return false, "'to' is not a string" end

If m.to is nil, then fallback "" is taken. Only if m.to is a table, bool or number this would return "not a string". Even if m.to is not nil but only "", where would it send to?

For my reference 7898