Fix empty string checks. (#5390)

There's no need to call the len function when checking for an empty
string.

dgraph/cmd/zero/raft.go

@@ -303,7 +303,7 @@ func (n *node) applyProposal(e raftpb.Entry) (string, error) {
 	if err := p.Unmarshal(e.Data); err != nil {
 		return p.Key, err
 	}
-	if len(p.Key) == 0 {
+	if p.Key == "" {
 		return p.Key, errInvalidProposal
 	}
 	span := otrace.FromContext(n.Proposals.Ctx(p.Key))

dgraph/cmd/zero/run.go

@@ -211,7 +211,7 @@ func run() {
 	if opts.bindall {
 		addr = "0.0.0.0"
 	}
-	if len(opts.myAddr) == 0 {
+	if opts.myAddr == "" {
 		opts.myAddr = fmt.Sprintf("localhost:%d", x.PortZeroGrpc+opts.portOffset)
 	}
 

dgraph/cmd/zero/zero.go

@@ -434,7 +434,7 @@ func (s *Server) Connect(ctx context.Context,
 		}
 		return cs, err
 	}
-	if len(m.Addr) == 0 {
+	if m.Addr == "" {
 		return &emptyConnectionState, errors.Errorf("NO_ADDR: No address provided: %+v", m)
 	}
 
@@ -568,7 +568,7 @@ func (s *Server) ShouldServe(
 	ctx, span := otrace.StartSpan(ctx, "Zero.ShouldServe")
 	defer span.End()
 
-	if len(tablet.Predicate) == 0 {
+	if tablet.Predicate == "" {
 		return resp, errors.Errorf("Tablet predicate is empty in %+v", tablet)
 	}
 	if tablet.GroupId == 0 && !tablet.ReadOnly {

posting/index.go

@@ -403,7 +403,7 @@ func (txn *Txn) addMutationHelper(ctx context.Context, l *List, doUpdateIndex bo
 // AddMutationWithIndex is addMutation with support for indexing. It also
 // supports reverse edges.
 func (l *List) AddMutationWithIndex(ctx context.Context, edge *pb.DirectedEdge, txn *Txn) error {
-	if len(edge.Attr) == 0 {
+	if edge.Attr == "" {
 		return errors.Errorf("Predicate cannot be empty for edge with subject: [%v], object: [%v]"+
 			" and value: [%v]", edge.Entity, edge.ValueId, edge.Value)
 	}

query/query.go

@@ -1961,7 +1961,7 @@ func ProcessGraph(ctx context.Context, sg, parent *SubGraph, rch chan error) {
 				return sg.DestUIDs.Uids[i] < sg.DestUIDs.Uids[j]
 			})
 		}
-	case len(sg.Attr) == 0:
+	case sg.Attr == "":
 		// This is when we have uid function in children.
 		if sg.SrcFunc != nil && sg.SrcFunc.Name == "uid" {
 			// If its a uid() filter, we just have to intersect the SrcUIDs with DestUIDs