Return only populated documents
Hey, let's say I want to get launches where the rocket is falcon 9.  So I would query launches, populate 'rocket' field and then check the rocket.name, right? AFAIK i can't use $where on populated fields but mongoose gives you `match` property and yeah it works, the only problem is that in response I get both populated(based on the given condition) and unpopulated documents(e.g. `rocket: null`), and I want the response to be just launches where `rocket` field is populated. 

Please tell me if I'm missing something, any help is appreciated.