fix(motion_velocity_planner): prevent crash with the downsampling function

[maxime-clem]

Description

A bug in the downsampling function of the motion_velocity_planner could cause crashes. This PR fixes it.

Related links

Private Links:

• TIER IV Evaluator link

How was this PR tested?

Psim Evaluator

Notes for reviewers

None.

Interface changes

None.

Effects on system behavior

None.

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 11.20%. Comparing base (507e3f4) to head (c1434b7). Report is 173 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #7727 +/- ## =========================================== - Coverage 14.84% 11.20% -3.65% =========================================== Files 1999 196 -1803 Lines 139163 17012 -122151 Branches 43716 3068 -40648 =========================================== - Hits 20661 1906 -18755 + Misses 95731 14745 -80986 + Partials 22771 361 -22410 ``` | [Flag](https://app.codecov.io/gh/autowarefoundation/autoware.universe/pull/7727/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=autowarefoundation) | Coverage Δ | | |---|---|---| | [differential](https://app.codecov.io/gh/autowarefoundation/autoware.universe/pull/7727/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=autowarefoundation) | `11.20% <100.00%> (?)` | | | [total](https://app.codecov.io/gh/autowarefoundation/autoware.universe/pull/7727/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=autowarefoundation) | `?` | |

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