logfiles with no airflow records give IndexError

> <root>/cbtb/cbtb/logs/process_logs.pyc in read_air_data(cname, pth, droptail)
>     111 
>     112     if droptail:
> --> 113         air_lines.pop()
>     114     return air_lines
>     115 
> 
> IndexError: pop from empty list

The droptail branch should also test whether the list is non-empty. 
This fix needs to be applied for all field types