avoid warnings in vertex truth match tool in case there are no vertices to match

The InDetVertexTruthMatchTool::match method currently throws a WARNING if after checking the vertices in the event it has not found at least one case of an attached track particle collection. Unfortunately, it also does so if there are no vertices to match in the first place (for example, on single particle MC), in which case this check fails by construction.

This MR makes the tool exit without complaining in case there are no vertices to match (either empty collection or just a dummy vertex), avoiding the warning as well as skipping execution of any downstream code.

@jojungge, thank you for the nice picture of Jürgen Habermas in !32488 (merged)!

Tagging @npetters for information