`Morphology` doesn't like empty data

(b4py9) robin@LAPTOP-JF6T3PSU:~/git/bsb-hdf5$ python -m unittest test.test_interface.TestPlacementSet.test_label_filter
/home/robin/.pyenv/versions/b4py9/lib/python3.9/site-packages/mpilock/__init__.py:295: UserWarning: Exception during write lock. Deadlock might occur if you use `.collect`.
E
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ERROR: test_label_filter (test.test_interface.TestPlacementSet)
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Traceback (most recent call last):
  File "/mnt/c/Users/pwd06/git/bsb/bsb/unittest/engines.py", line 373, in test_label_filter
    self.network.morphologies.save("testA", Morphology([Branch([], [])]))
  File "/mnt/c/Users/pwd06/git/bsb-hdf5/bsb_hdf5/morphology_repository.py", line 122, in save
    morphology.optimize()
  File "/mnt/c/Users/pwd06/git/bsb/bsb/morphologies/__init__.py", line 610, in optimize
    points[ptr:nptr] = branch.points
ValueError: could not broadcast input array from shape (0,) into shape (0,3)

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Ran 1 test in 0.135s

FAILED (errors=1)
sys:1: ResourceWarning: unclosed file <_io.TextIOWrapper name=1 encoding='UTF-8'>
(b4py9) robin@LAPTOP-JF6T3PSU:~/git/bsb-hdf5$ python -m unittest test.test_interface.TestPlacementSet.test_label_filter
/home/robin/.pyenv/versions/b4py9/lib/python3.9/site-packages/mpilock/__init__.py:295: UserWarning: Exception during write lock. Deadlock might occur if you use `.collect`.
E
======================================================================
ERROR: test_label_filter (test.test_interface.TestPlacementSet)
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Traceback (most recent call last):
  File "/mnt/c/Users/pwd06/git/bsb/bsb/unittest/engines.py", line 373, in test_label_filter
    self.network.morphologies.save("testA", Morphology([Branch(np.empty((0,3)), [])]))
  File "/mnt/c/Users/pwd06/git/bsb-hdf5/bsb_hdf5/morphology_repository.py", line 152, in save
    root.attrs["meta:ldc"] = np.min(morphology._shared._points, axis=0)
  File "<__array_function__ internals>", line 180, in amin
  File "/home/robin/.pyenv/versions/b4py9/lib/python3.9/site-packages/numpy/core/fromnumeric.py", line 2916, in amin
    return _wrapreduction(a, np.minimum, 'min', axis, None, out,
  File "/home/robin/.pyenv/versions/b4py9/lib/python3.9/site-packages/numpy/core/fromnumeric.py", line 86, in _wrapreduction
    return ufunc.reduce(obj, axis, dtype, out, **passkwargs)
ValueError: zero-size array to reduction operation minimum which has no identity

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dbbs-lab / bsb-core

`Morphology` doesn't like empty data #586
