fix traceback in sidewinder when arpeggiation is enabled

Was getting a traceback with arpeggiation enabled in sidewinder.py.  In line 120, in the case of an empty set for self._down_keys, the set([]) was stored in send_strokes due to the short-circuiting of python's 'and' operator.  The subsequent 'send_strokes &= ...' in line 123 failed due to mismatched types (set and bool).  The fix is to ensure a boolean value is always stored in send_strokes.