pybind/mgr/balancer: fix sanity check against empty weight-set

There are paths get_compat_weight_set_weights() will return a
empty dict, not None.

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>

src/pybind/mgr/balancer/module.py

@@ -674,7 +674,7 @@ def do_crush_compat(self, plan):
 
         # get current compat weight-set weights
         orig_ws = self.get_compat_weight_set_weights(ms)
-        if orig_ws is None:
+        if not orig_ws:
             return False
         orig_ws = { a: b for a, b in orig_ws.iteritems() if a >= 0 }