feat(behavior_path_planner): add error handling for null output

[TakaHoribe]

Description

Add an error handling when the module output is null.

Without this code, some modules may die due to invalid memory access.

Tests performed

Run psim

Effects on system behavior

Not applicable.

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[codecov]

Codecov Report

Patch coverage: 12.50% and project coverage change: +0.03% 🎉

Comparison is base (04ea73c) 15.03% compared to head (093113a) 15.06%.

❗ Current head 093113a differs from pull request most recent head a73bde2. Consider uploading reports for the commit a73bde2 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4352      +/-   ##
==========================================
+ Coverage   15.03%   15.06%   +0.03%     
==========================================
  Files        1508     1512       +4     
  Lines      103879   106302    +2423     
  Branches    31730    33577    +1847     
==========================================
+ Hits        15614    16011     +397     
- Misses      71233    72920    +1687     
- Partials    17032    17371     +339     

Flag	Coverage Δ		*Carryforward flag
differential	13.51% <12.50%> (?)
total	15.09% <ø> (+0.05%)	⬆️	Carriedforward from 0399032

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

Files Changed	Coverage Δ
.../include/behavior_path_planner/planner_manager.hpp	18.86% <ø> (ø)
...ning/behavior_path_planner/src/planner_manager.cpp	19.93% <12.50%> (+0.15%)	⬆️

... and 39 files with indirect coverage changes

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[kosuke55]

the PR title is wrong?