What could be a problem if in C++ program VideoWriter.write() doesn't write anything. IsOpened() returns true. All parameters in VideoWriter.open() are surely correct, and frame size which is tried to write is same as given in VideoWriter.open().
Format is webm. CV_FOURCC('V', 'P', '9', '0')
No any runtime-errors. After VideoWriter.release() file size is 419 bytes and it contains proper EBML header (checked with mkvinfo), which video duration 0.