return empty result when srcVertices ids is empty.
return empty result when srcVertices ids is empty.

jongwook/incubator-s2graph,daewon/incubator-s2graph,SteamShon/incubator-s2graph,daewon/incubator-s2graph,SteamShon/incubator-s2graph,jongwook/incubator-s2graph,SteamShon/incubator-s2graph