Create JetInputExternal definition for HLT topoclusters

In !70919 (merged) I moved to setting HLT topoclusters as input prerequisites for HLT PFlow to ease passing around the information, but this resulted in breakage when HLT PFlow chains were configured standalone, without HLT Topocluster jets (e.g. for preselection). That was caught by the ART test for the standalone MET slice.

I don't think the bug report has been generated yet, but can link when this is posted. Thanks @jmasik for catching this.