[Jsk_Lib] skip update func when servo_num == 0

[makabe0510]

What is this

Skip servo_.update() func when no kondo servo is connected as same as dynamixel.

Details

add below code to the head of KondoServo::update() function

if (servo_num_ == 0) return;

TODO

• test communication is not terminated with spinal_v2 board

[tongtybj]

@makabe0510

Thx for your commitment.
This feature might be merged in #742 where spinal will automatically detect whehter servos (either kondo or dynamixel) are directly connected. If there are no servos, spinal will not call the update func:
https://github.com/jsk-ros-pkg/jsk_aerial_robot/pull/742/files#diff-b272152f98e68aa4075dafdedc9bad160d0e9406dfc59bc7b14f55ba31d3d6b0R1356-R1359