[master] Always use root:// in HistogramMergingAgent
Needs: https://github.com/DIRACGrid/DIRAC/pull/7632
BEGINRELEASENOTES
*ProductionManagement FIX: Always use root:// in HistogramMergingAgent
ENDRELEASENOTES
Needs: https://github.com/DIRACGrid/DIRAC/pull/7632
BEGINRELEASENOTES
*ProductionManagement FIX: Always use root:// in HistogramMergingAgent
ENDRELEASENOTES