Prevent no attribute error when using Mtcnn

[MattPChoy]

What has been done

When running this code with an image with no faces, a TypeError was encountered. Using short-circuit evaluation a condition was added to prevent this exception from occurring again.

Traceback (most recent call last):
  File "XXXXXXX.py", line 65, in find_face
    enforce_detection=enforce_detection)
  File "/home/YYYYYY/anaconda3/envs/facedetection/lib/python3.7/site-packages/deepface/DeepFace.py", line 451, in extract_faces
    grayscale=grayscale,
  File "/home/YYYYYY/anaconda3/envs/facedetection/lib/python3.7/site-packages/deepface/modules/detection.py", line 57, in extract_faces
    align=align,
  File "/home/YYYYYY/anaconda3/envs/facedetection/lib/python3.7/site-packages/deepface/commons/functions.py", line 178, in extract_faces
    face_objs = DetectorWrapper.detect_faces(detector_backend, img, align)
  File "/home/YYYYYY/anaconda3/envs/facedetection/lib/python3.7/site-packages/deepface/detectors/DetectorWrapper.py", line 78, in detect_faces
    return face_detector.detect_faces(img=img, align=align)
  File "/home/YYYYYY/anaconda3/envs/facedetection/lib/python3.7/site-packages/deepface/detectors/FastMtCnn.py", line 47, in detect_faces
    if len(detections[0]) > 0:
TypeError: object of type 'NoneType' has no len()

How to test

make lint && make test

[serengil]

Good point! Thank you for your contribution.