fix(behavior_velocity): fix segfo at for unstable subscriber

[taikitanaka3]

Signed-off-by: tanaka3 ttatcoder@outlook.jp

Description

related to #1297
for unstable clock velocity buffer can be zero and that can cause segmentation fault
before this PR

[openscenario_interpreter_node-3] [component_container_mt-44] ERROR time_diff : 45.2335 velocity_buffer size: 2
[openscenario_interpreter_node-3] [component_container_mt-44] ERROR time_diff : 45.2283 velocity_buffer size: 1
[openscenario_interpreter_node-3] [component_container_mt-44] ERROR time_diff : 1.65752e+09 velocity_buffer size: 0
[openscenario_interpreter_node-3] [component_container_mt-44] ERROR time_diff : 1.65752e+09 velocity_buffer size: 18446744073709551615

-> segmenation fault

after this PR

experiment-2022-07-11_15.27.51.mp4

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[codecov]

Codecov Report

Merging #1300 (27275d8) into main (44c4bd1) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@           Coverage Diff            @@
##            main   #1300      +/-   ##
========================================
- Coverage   9.48%   9.46%   -0.03%     
========================================
  Files       1096    1096              
  Lines      76293   76468     +175     
  Branches   16990   17110     +120     
========================================
+ Hits        7239    7240       +1     
- Misses     62440   62590     +150     
- Partials    6614    6638      +24     

Flag	Coverage Δ		*Carryforward flag
differential	4.95% <0.00%> (?)
total	9.47% <0.00%> (ø)		Carriedforward from 336b92a

*This pull request uses carry forward flags. Click here to find out more.

Impacted Files	Coverage Δ
planning/behavior_velocity_planner/src/node.cpp	0.76% <0.00%> (+0.30%)	⬆️
...city_planner/src/scene_module/blind_spot/scene.cpp	0.00% <0.00%> (ø)

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[TomohitoAndo]

@taikitanaka3
According to the ROS2 Code style, you should always use braces for if and else.
https://docs.ros.org/en/rolling/The-ROS2-Project/Contributing/Code-Style-Language-Versions.html#always-use-braces

Also, I think it would be more simple by writing like this. (early return)

    if (planner_data_.velocity_buffer.empty()) {
      break;
    }

    // Remove old data
    planner_data_.velocity_buffer.pop_back();

[TomohitoAndo]

I confirmed this works in planning simulator.