Fix MIX success log to be printed only when success at least 1 server

When MIX (get_diff) was failed on all the servers, `success to get_diff from` message is always printed.
This is redundant.

```
W0514 11:55:36.150894 19321 linear_mixer.cpp:462] get_diff failed at 192.168.122.211:9199 : connect failed
I0514 11:55:36.150946 19321 linear_mixer.cpp:474] success to get_diff from []
W0514 11:55:36.151079 19321 linear_mixer.cpp:529] mix failed :no diff available
```