Remove empty field check in Inspect()

The check for field existence is not necessary; since we
add the `_id` field to every response then at the very
least that field will be returned.

This check was preventin a query like `select _id from ...`
from returning any results.

server/grpc.go

@@ -93,11 +93,6 @@ func (h grpcHandler) Inspect(req *pb.InspectRequest, stream pb.Pilosa_InspectSer
 		}
 	}
 
-	// If there are no matching fields, then don't return any records.
-	if len(fields) == 0 {
-		return nil
-	}
-
 	limit := req.Limit
 	if limit == 0 {
 		limit = defaultLimit