Patch to hard scatter collection maker for single particles
Single particle samples have a particularly unusual truth record: they often have a 'hard scatter vertex' that doesn't have any incomming particles; they are sparse; they may have one or no truth particles in an event.
The hard scatter collection maker was set up with a very pp-collision-centric view of the truth collection, and the assumptions made in that collection maker were simply not valid for single particle samples. This led to crashes like those reported in ATLASDPD-1564
This patch should solve the problem by removing those assumptions. It's been tested to solve the crash, and should give identical output for the usual complex event records.