Note that this is not PG, Raijin is a new implementation and is not built on Postgres.
Otherwise the question is valid. Raijin does not store NULL values though. In the above example a specific field (column) was selected and that's why it appears as null. Perhaps the output would be more appropriate this way then (which is a modification for us to consider):
{"foo":"bar"}
{"foo":"bar2","intfield":42}
Note that when you select the full record with the star it does not return null values:
select * from tbl;
--
{"_id":1,"eventtime":"2016-07-20 10:37:12","foo":"bar","whatever":"xx"}
{"_id":2,"eventtime":"2016-07-20 10:38:22","foo":"bar2","intfield":42}
The undef/exists thing can get a bit confusing when you are mapping this into SQL.
