fix(storage): add a safety check for readhandle (#11549)

By running some tests observed that default ReadObjectSpec looks like: 
`read_object_spec:{bucket:"projects/_/buckets/bucket-name"  object:"obj-name"  read_handle:{}}`

Which is not correct as we pass nil read_handle. Added an check in NewRangeReader and NewMultiRangeDownloader to only pass read_handle in read_object_spec when it is not nil.

Reason: Default value of readHandle that we attach to objectHandle will be []( same as default value of byte array).

In case nil read_handle is passed by user we would already be doing the slow auth hence adding the check should not be a issue in that case too.

Author: shubham-diwakar

storage/grpc_client.go

@@ -1092,7 +1092,7 @@ func (c *grpcStorageClient) NewMultiRangeDownloader(ctx context.Context, params
 		r.Generation = params.gen
 	}
 
-	if params.handle != nil {
+	if params.handle != nil && len(*params.handle) != 0 {
 		r.ReadHandle = &storagepb.BidiReadHandle{
 			Handle: *params.handle,
 		}
@@ -1107,7 +1107,7 @@ func (c *grpcStorageClient) NewMultiRangeDownloader(ctx context.Context, params
 		if err := applyCondsProto("grpcStorageClient.BidiReadObject", params.gen, params.conds, r); err != nil {
 			return nil, nil, err
 		}
-		if readHandle != nil {
+		if len(readHandle) != 0 {
 			req.GetReadObjectSpec().ReadHandle = &storagepb.BidiReadHandle{
 				Handle: readHandle,
 			}
@@ -1526,7 +1526,7 @@ func (c *grpcStorageClient) NewRangeReader(ctx context.Context, params *newRange
 	if err := applyCondsProto("gRPCReader.NewRangeReader", params.gen, params.conds, spec); err != nil {
 		return nil, err
 	}
-	if params.handle != nil {
+	if params.handle != nil && len(*params.handle) != 0 {
 		spec.ReadHandle = &storagepb.BidiReadHandle{
 			Handle: *params.handle,
 		}