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.

(cherry picked from commit 428f3bd)

worker/groups.go

@@ -445,8 +445,6 @@ 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]
@@ -464,7 +462,7 @@ 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