Fail fast if inputs are missing on positive decisions in SelReportsMaker

When SelReportsMaker is used in the HLTControlFlowMgr as a 'barrier algorithm' it is passed null pointers when an optional data dependency is not present. It is then the responsibility of the application to ensure the inputs exist on a positive trigger decision. This change causes SelReportsMaker to fail very loudly if that condition is not met.

See Moore#145 (closed) for more context.

Requires LHCb!2954 (merged). Required for Moore!759 (merged).