use items with "empty" values (e.g. location_code="") in string represen...

...tation and compare against None

For example this is confusing in my opinion:
```python
In [1]: from obspy.core.event import WaveformStreamID

In [2]: WaveformStreamID(network_code="BW", location_code="")
Out[2]: WaveformStreamID(network_code='BW')
```

Opinions?
Some (doc)tests would need adapting..