Empty ParticleGroup unable to be saved to HDF5

[ken-lauer]

This fails here:

File ~/Repos/openPMD-beamphysics/beamphysics/particles.py:1467, in ParticleGroup.write(self, h5, name)
   1464 else:
   1465     g = h5
-> 1467 write_pmd_bunch(g, self, name=name)

File ~/Repos/openPMD-beamphysics/beamphysics/writers.py:79, in write_pmd_bunch(h5, data, name)
     76     u = pg_units(key)
     78     # Write
---> 79     write_component_data(g, g2_name, data[key], unit=u)
     81 # Optional id. This does not have any units.
     82 if "id" in data:

File ~/Repos/openPMD-beamphysics/beamphysics/writers.py:137, in write_component_data(h5, name, data, unit)
    128 """
    129 Writes data to a dataset h5[name]
    130
   (...)    134
    135 """
    136 # Check for constant component
--> 137 dat0 = data.flat[0]
    139 if np.all(data == dat0):
    140     g = h5.create_group(name)