Histogram merging: Fix HLT use-case

Walter Lamplrequested to merge
wlampl/athena:HistMergeNoRunDir into 24.0

While testing !78484 (merged), I overlooked one use-case that surfaced now as ADCSUPPORT-5725.

With this MR, the DQHistogramMerge job can deal with input files that contain no "run_NNNNN" directory at top level and that contain no metadata trees in the indiviual sub-directories.

Closes ADCSUPPORT-5725

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