rest_qos: Avoid None when deleting OVSDB addr
rest_qos: Avoid None when deleting OVSDB addr

Currently, rest_qos.py will raise AttributeError when deleting OVSDB
server address because rest_qos.py will try to split the given address
string but the address is None when deleting.

This patch checks if the given address is None or not before the string
manipulation and fixes this problem.

Signed-off-by: IWASE Yusuke <bb80e84cc9f5a8f01385872ee89032beb5d4eba7@gmail.com>
Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

lagopus/ryu-lagopus-ext,fujita/ryu,fujita/ryu,osrg/ryu,fujita/ryu,iwaseyusuke/ryu,osrg/ryu,fujita/ryu,iwaseyusuke/ryu,lagopus/ryu-lagopus-ext,lagopus/ryu-lagopus-ext,fujita/ryu,osrg/ryu,iwaseyusuke/ryu,iwaseyusuke/ryu,iwaseyusuke/ryu,lagopus/ryu-lagopus-ext,lagopus/ryu-lagopus-ext,osrg/ryu,osrg/ryu