DNN: YOLOv4-tiny no detections for OpenCV/OpenCL target

see #17743

```
[ RUN      ] Test_Darknet_nets.YOLOv4_tiny/0, where GetParam() = OCV/OCL
/build/precommit_opencl_linux/3.4/opencv/modules/dnn/test/test_common.impl.hpp:99: Failure
Value of: testClassIds.empty()
  Actual: true
Expected: false
No detections
Google Test trace:
/build/precommit_opencl_linux/3.4/opencv/modules/dnn/test/test_darknet_importer.cpp:616: batch size 1
```