paths of length zero not added to network

Adding paths of length zero to a path object, e.g:
```
p = pp.Paths()
for i in range(10):
    p.add_path(tuple([i]))
```
When then generate a network path:
```
n = pp.Network.from_paths(p)
```
we get an empty network instead of one of isolated nodes:
```
n.nodes
defaultdict(dict, {})
```