Data conversion with "empty" files is not robust

[BenMoon]

If quasi empty files get converted to hdf5, the centroiding process crashes:

Exception in thread Thread-10:
Traceback (most recent call last):
  File "/usr/lib/python3.7/threading.py", line 917, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.7/threading.py", line 865, in run
    self._target(*self._args, **self._kwargs)
  File "/home/cfelcmi/timepix/pymepix-viewer/pymepixviewer/dialogs/postprocessing.py", line 200, in run_post_processing_threaded
    progress_callback=self.updateProgressBar,
  File "/home/cfelcmi/timepix/pymepix/pymepix/post_processing.py", line 14, in run_post_processing
    file_sampler.run()
  File "/home/cfelcmi/timepix/pymepix/pymepix/processing/rawfilesampler.py", line 268, in run
    self.push_data()
  File "/home/cfelcmi/timepix/pymepix/pymepix/processing/rawfilesampler.py", line 153, in push_data
    self.__calculate_and_save_centroids(*result)
  File "/home/cfelcmi/timepix/pymepix/pymepix/processing/rawfilesampler.py", line 163, in __calculate_and_save_centroids
    centroids = self.centroid_calculator.process(event_data)
  File "/home/cfelcmi/timepix/pymepix/pymepix/processing/logic/centroid_calculator.py", line 120, in process
    return self.__centroid_chunks_to_centroids(centroids_in_chunks)
  File "/home/cfelcmi/timepix/pymepix/pymepix/processing/logic/centroid_calculator.py", line 169, in __centroid_chunks_to_centroids
    return np.concatenate(list(chunks), axis=1)
ValueError: zero-dimensional arrays cannot be concatenated

Add a check with if len(chunks) >0 to __centroid_chunks_to_centroids in centroid_calculator.py probably already solves the problem.