CAF POT Histogram (SBN 2025A)

[lynnt20]

Description

Identical to #597 (this PR is for 2025A spring production release)

When the input artroot file(s) have 0 art events, subrun-level information (such as POT) is not properly saved in rec.hdr.* branches since they are only filled during call to art event. In order to fully persist the POT information, the POT histogram TotalPOT needs to be saved even if there are no art events in the file. Also documented in docb44065.

Note: recTree still seems to be created in the output caf files, but the genie tree is not.

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