Fix to PR #152

I forgot the edge case when `atom.element is None`, which happens for vsites in the topology.

```
from simtk.openmm import app
p = app.PDBFile('<opennmm>/wrappers/python/simtk/openmm/app/data/tip5p.pdb')
app.PDBFile.writeFile(p.topology, p.positions, open('tip5p', 'w')
```
will throw `AttributeError: 'NoneType' object has no attribute 'symbol'`