title: How to fix "ValueError: need at least one array to concatenate" error
body: <p>Following up from here: <a href="https://stackoverflow.com/questions/25349178/calculating-percentage-of-bounding-box-overlap-for-image-detector-evaluation/26386665#26386665">Calculating percentage of Bounding box overlap, for image detector evaluation</a>, I'm getting an error at this line:</p>
  
  <pre><code>poly_clipped = poly.clip_to_bbox(clip_rect).to_polygons()[0]
  </code></pre>
  
  <p>This is the error:</p>
  
  <pre><code> File "C:\work_asaaki\code\detection.py", line 32, in clip_boxes
      poly_clipped = poly.clip_to_bbox(clip_rect).to_polygons()[0]
    File "C:\Anaconda\lib\site-packages\matplotlib\path.py", line 909, in clip_to_bbox
      return self.make_compound_path(*paths)
    File "C:\Anaconda\lib\site-packages\matplotlib\path.py", line 328, in make_compound_path
      vertices = np.vstack([x.vertices for x in args])
    File "C:\Anaconda\lib\site-packages\numpy\core\shape_base.py", line 228, in vstack
      return _nx.concatenate([atleast_2d(_m) for _m in tup], 0)
  ValueError: need at least one array to concatenate
  </code></pre>
  
  <p>This doesn't always happen, it happens based on specific sets of polygons... What I'm trying to understand is when exactly does it not work? And how can I solve the issue?</p>