issue when no pixel size exists

If there is not pixel size then nabu ends with the following error:

  File "/media/payno/830add5c-f5d7-4db2-875e-03be4f0a67f0/dev/tomography/nabu/nabu/pipeline/fullfield/processconfig.py", line 372, in _build_processing_steps
    rec_options["pixel_size_cm"] = dataset_info.pixel_size * 1e-4 # pix size is in microns
  File "/media/payno/830add5c-f5d7-4db2-875e-03be4f0a67f0/dev/tomography/nabu/nabu/resources/dataset_analyzer.py", line 288, in pixel_size
    return self.dataset_scanner.pixel_size * 1e6
TypeError: unsupported operand type(s) for *: 'NoneType' and 'float'

Having a more 'meaningful' error would be better for non advanced users