Handle `None` in `ForEachObject.write`

I ran into the following problem when running write() with write_all=True. The problem is connected to Nones in the ForEachMosa instance. I think ForEachObject.write needs some modification to handle those correctly.

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-33af2829abe5> in <module>
      3 instrument = lisainstrument.Instrument(size=size+skipped, orbits='data/esa_orbits.h5')
      4 instrument.disable_all_noises(but='laser')
----> 5 instrument.write('data/laser_only_esa.h5', write_all=True)

~/20190815_PhD_AEI/projects/instrument/lisainstrument/instrument.py in write(self, output, mode, write_all)
   1472 
   1473             logger.debug("Writing laser noise to '%s'", output)
-> 1474             self.laser_noises.write(hdf5, 'laser_noises')
   1475 
   1476             logger.debug("Writing clock noise to '%s'", output)

~/20190815_PhD_AEI/projects/instrument/lisainstrument/containers.py in write(self, hdf5, dataset)
     70             if numpy.isscalar(value):
     71                 continue
---> 72             if size != 1 and len(value) != size:
     73                 raise ValueError(f"incompatible sizes in dictionary '{size}' and '{len(size)}'")
     74             size = max(size, len(value))

TypeError: object of type 'NoneType' has no len()