Return nil instead of emptyTablet in groupi.Tablet() (#5469)

groupi.Tablet returns error and empty Tablet when error is returned while querying zero for
Tablet serving info. Allocation of space by empty Tablet shows up in alloc_space profile.

         .          .    463:func (g *groupi) Tablet(key string) (*pb.Tablet, error) {
   16.22GB    16.22GB    464:	emptyTablet := pb.Tablet{}
         .          .    465:
         .          .    466:	// TODO: Remove all this later, create a membership state and apply it
         .          .    467:	g.RLock()
         .          .    468:	tablet, ok := g.tablets[key]
         .          .    469:	g.RUnlock()
         .          .    470:	if ok {
         .          .    471:		return tablet, nil
         .          .    472:	}
         .          .    473:
         .          .    474:	// We don't know about this tablet.
         .          .    475:	// Check with dgraphzero if we can serve it.
         .          .    476:	pl := g.connToZeroLeader()
         .          .    477:	zc := pb.NewZeroClient(pl.Get())
         .          .    478:
         .          .    479:	tablet = &pb.Tablet{GroupId: g.groupId(), Predicate: key}
         .     1.07MB    480:	out, err := zc.ShouldServe(context.Background(), tablet)
         .          .    481:	if err != nil {
         .          .    482:		glog.Errorf("Error while ShouldServe grpc call %v", err)
         .          .    483:		return &emptyTablet, err
         .          .    484:	}

This can be avoided. We can return nil instead of emptyTablet and save allocations here. Caller
function always checks for error first before accessing tablet information.

worker/groups.go

@@ -461,8 +461,6 @@ func (g *groupi) ServesTablet(key string) (bool, error) {
 
 // Do not modify the returned Tablet
 func (g *groupi) Tablet(key string) (*pb.Tablet, error) {
-	emptyTablet := pb.Tablet{}
-
 	// TODO: Remove all this later, create a membership state and apply it
 	g.RLock()
 	tablet, ok := g.tablets[key]
@@ -480,7 +478,7 @@ func (g *groupi) Tablet(key string) (*pb.Tablet, error) {
 	out, err := zc.ShouldServe(context.Background(), tablet)
 	if err != nil {
 		glog.Errorf("Error while ShouldServe grpc call %v", err)
-		return &emptyTablet, err
+		return nil, err
 	}
 
 	// Do not store tablets with group ID 0, as they are just dummy tablets for