refactor(motion_velocity_smoother): remove unneccesary optional

[TakaHoribe]

Description

The boost::optional for filters used in the motion_velocuty_smoother gets none only when the path is empty.
This PR moves the empty check out of the filters and changes the return value to the actual value.

Pre-review checklist for the PR author

The PR author must check the checkboxes below when creating the PR.

• I've confirmed the contribution guidelines.
• The PR follows the pull request guidelines.

In-review checklist for the PR reviewers

The PR reviewers must check the checkboxes below before approval.

• The PR follows the pull request guidelines.

Post-review checklist for the PR author

The PR author must check the checkboxes below before merging.

• There are no open discussions or they are tracked via tickets.

After all checkboxes are checked, anyone who has write access can merge the PR.

[codecov]

Codecov Report

Base: 11.61% // Head: 11.58% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (a20c7af) compared to base (ace34c6).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2811      +/-   ##
==========================================
- Coverage   11.61%   11.58%   -0.03%     
==========================================
  Files        1319     1319              
  Lines       92085    92313     +228     
  Branches    24408    24492      +84     
==========================================
  Hits        10692    10692              
- Misses      70227    70450     +223     
- Partials    11166    11171       +5     

Flag	Coverage Δ		*Carryforward flag
differential	3.42% <0.00%> (?)
total	11.61% <0.00%> (ø)		Carriedforward from ace34c6

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

Impacted Files	Coverage Δ
...y_planner/include/utilization/trajectory_utils.hpp	0.00% <0.00%> (ø)
...ity_smoother/src/motion_velocity_smoother_node.cpp	0.00% <0.00%> (ø)
..._smoother/analytical_jerk_constrained_smoother.cpp	0.00% <0.00%> (ø)
...n_velocity_smoother/src/smoother/smoother_base.cpp	0.00% <0.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[purewater0901]

LGTM

[rej55]

LGTM