Empty _source load

Why elasticsearch is allowing to load a null document (means empty _source)?

{
"_index" : "null_index",
"_type" : "_doc",
"_id" : "1",
"_score" : 1.0,
"_source" : { }
}

It is returning, what has been indexed previously, there are no sanity checks for this. It also means, there will be no real way to query for this document.