fix: Add missing initialisation for SplineSection
requested to merge mkellerer/openscenario1_engine:fix_missing_spline_initialisation_for_speedaction into main
Description
SpeedAction calculates "incorrect" acceleration and velocity values due to not initialised polynomial coefficients of as mantle_api::SplineSection. The polynomial coefficients are not initialised as mantle_api::SplineSection is a POD and therefore is
not default initialised during construction.
Default initialisation of mantle_api::SplineSection got removed with mantle-api!189 (merged)