fix(dgraph): Fix segmentation fault in draft.go (#5798) (#5860)

Use the GetXXX methods of the generated protobufs to correctly handle
the case where no predicate hints are available.

Fixes DGRAPH-1753.

worker/draft.go

@@ -394,7 +394,7 @@ func (n *node) applyMutations(ctx context.Context, proposal *pb.Proposal) (rerr
 	for attr, storageType := range schemaMap {
 		if _, err := schema.State().TypeOf(attr); err != nil {
 			hint := pb.Metadata_DEFAULT
-			if mutHint, ok := proposal.Mutations.Metadata.PredHints[attr]; ok {
+			if mutHint, ok := proposal.GetMutations().GetMetadata().GetPredHints()[attr]; ok {
 				hint = mutHint
 			}
 			if err := createSchema(attr, storageType, hint); err != nil {