!3789 (merged) caused a segfault in Boole DD4hep builds, because the removal of XmlParserSvc didn't take into account its uses at runtime, in particular in LbAppInit.
With this MR I add a protection to LbAppInit to avoid a segfault in case of configuration problems and I extend DD4hepSvc with the interface ICondDBInfo so that it can be used by LbAppInit to gather CondDB info in DD4hep builds.
I also added a unit test to check that the fix actually works.
/cc @sponce