Fix PostprocessCallback backup_condition for non-zero t_0 and restarts#1103

Copilot wants to merge 2 commits intomaintrixi-framework/TrixiParticles.jl:mainfrom

copilot/fix-postprocess-callback-conditiontrixi-framework/TrixiParticles.jl:copilot/fix-postprocess-callback-conditionCopy head branch name to clipboard