Only do pairwise energy conservation if there are two or more particles#6818

JustinRayAngus merged 2 commits intoBLAST-WarpX:developmentBLAST-WarpX/warpx:developmentfrom

dpgrote:fix_collision_pairwise_energy_conservationdpgrote/WarpX:fix_collision_pairwise_energy_conservationCopy head branch name to clipboard