Fix OpenCL device detection when some OpenCL platform has no devices

Fix OpenCL device detection when some OpenCL platform has no devices

It's not an error if some OpenCL platform has no devices. This makes
OpenCL device detection work correctly in the following scenario:

$ OPENCV_OPENCL_DEVICE=:GPU: ./opencv_test_dnn

OpenCV version: 4.1.2-dev
OpenCV VCS version: 4.1.2-80-g467748ee98-dirty
Build type: Debug
Compiler: /usr/bin/g++  (ver 7.4.0)
Parallel framework: pthreads
CPU features: SSE SSE2 SSE3 *SSE4.1 *SSE4.2 *FP16 *AVX *AVX2 *AVX512-SKX?
Intel(R) IPP version: ippIP AVX2 (l9) 2019.0.0 Gold (-) Jul 24 2018
OpenCL Platforms:
    AMD Accelerated Parallel Processing
    Portable Computing Language
        CPU: pthread-AMD Ryzen 7 2700X Eight-Core Processor (OpenCL 1.2 pocl HSTR: pthread-x86_64-pc-linux-gnu-znver1)
    NVIDIA CUDA
        dGPU: GeForce GTX 1080 (OpenCL 1.2 CUDA)
Current OpenCL device:
    Type = dGPU
    Name = GeForce GTX 1080
    Version = OpenCL 1.2 CUDA
    Driver version = 430.26