FractionalVolume with surface_fraction=0 does not work for 3d Region

[rgourdine]

Example:

dend1 = h.Section("dend1")
dend1.nseg = 4

cyt = rxd.Region(dend1.wholetree(), "i", geometry=rxd.FractionalVolume(volume_fraction=0.8), dx=0.25)
ca = rxd.Species(cyt, name="ca", charge=2, initial=1e-12)
dend1.L = 10
dend1.diam = 2

rxd.set_solve_type(domain=[dend1], dimension=3)
h.finitialize()
print(sum(ca.nodes(cyt).volume)

Expected behavior:
Not raising an exception

[adamjhn]

I think the reason we have not encountered this error before is in cyt the surface_fraction=0 but the nrn_region='i'. So at initialization we try to calculate how to scale the currents. The error in species.py occurs because surface_areas1d return 0 rather than a list.
@ramcdougal Should we throw an exception if surface_fraction=0 and nrn_region='i', or just add warning and avoid adding currents to that region?