Fix technical issue with write out of lhe preamble AGENE-2156
This MR fixes a technical detail in the algorithms used to hack LHE files while running in Powheg the generation of unsupported final states. The modification is necessary otherwise the code crashes in the latest version of the software.
The original MR for the implementation of the algorithms is !61671 (merged) and at the time this issue was not noticed since it was tested in a previous version of the software (rel 21).