Fixed an error when a handlist object does not have the 'empty' attribute

Added an extra check to see if the `empty` attribute exists before checking its value, in order to avoid the following error with the 1.0.9+8391 version of the SDK:

```
Traceback (most recent call last):
  File "/home/vlad/catkin_ws/install/lib/leap_motion/leap_interface.py", line 65, in on_frame
    if not frame.hands.empty:
  File "/home/vlad/LeapSDK/lib/Leap.py", line 745, in <lambda>
    __getattr__ = lambda self, name: _swig_getattr(self, HandList, name)
  File "/home/vlad/LeapSDK/lib/Leap.py", line 55, in _swig_getattr
    raise AttributeError(name)
AttributeError: empty
```