Output json path not always contains dirname

[banderlog]

Without this patch python3 ./voc2coco.py ../Annotations/ tmp.json will raise an exception:

Number of xml files: 1750
Traceback (most recent call last):
  File "./voc2coco.py", line 153, in <module>
    convert(xml_files, args.json_file)
  File "./voc2coco.py", line 133, in convert
    os.makedirs(os.path.dirname(json_file), exist_ok=True)
  File "/usr/lib/python3.6/os.py", line 220, in makedirs
    mkdir(name, mode)
FileNotFoundError: [Errno 2] No such file or directory: ''