Fixed length can be 0

In the test there was a not too many.

gpxpy/gpx.py

@@ -483,7 +483,7 @@ def speed_between(self, track_point):
         if not length:
             length = self.distance_2d(track_point)
 
-        if not seconds or length is not None:
+        if not seconds or length is None:
             return None
 
         return length / float(seconds)