Sweeping !1406 from master to devel: HistogramMergingAgent: set keys refPFN and refLFN with default values

Currently the CI in lhcb-simulation/SimDQData runs a script that adds the keys refPFN and refLFN to each new YAML file that is added.

Overwhelmingly, however, there is no appropriate reference so by default the script chooses the histograms from the request itself to be reference, to avoid breaking any code that may assume the presence of these keys. This results in a lot of commits which add zero useful information, which could be reduced if the default values were set when the file is written to begin with.

BEGINRELEASENOTES

*ProductionManagement

CHANGE: set keys refPFN and refLFN with default values in HistogramMergingAgent

ENDRELEASENOTES