Do not try to process empty inputs.

Details 12176

remove from DNN module headers

	* Keep image aspect ratio in resize called in grad_system and
weak_order
	* Bug correction in loop inside Decolor::gradvector

* core(test): merge hang test

* core(merge/split): fix intrin optimization

fixed call open(int id) failed

All vx_call() must be wrapped into own simd128/simd256/simd512 namespace

```
namespace CV__SIMD_NAMESPACE {
... vx_call declaration is here ...
}
```

- use '3.4.x' cache name for current maintenance series (there are no serious changes between releases)
- message is shown only once during creation of new cache directory
- use OPENCV_CACHE_SHOW_CLEANUP_MESSAGE=0 to hide this warning

imgproc: update cornerSubPix documentation