Issue with loading gps data, loader expects a not empty 'direction'

I think the issue lies in `io.py` in the schema definition in line 230

```
'direction': (not_callandtext and r.direction is None) or r.direction in ['in', 'out'],
```

should be

```
'direction': (not_callandtext and r.direction in [None, '']) or r.direction in ['in', 'out'],
```
