Fixed code checking counters/Histogram for when no ref exists
The histogram checking introduced in !1661 (merged) had a bug that is only visible when no ref is present for a counter which appears in the output, and was causing affected nightly QMTests to fail with the cryptic error
ERROR:root:Exception caught when trying to instantiate qmt test python object
ERROR:root:not all arguments converted during string formatting
This MR fixes it. @jonrob FYI