histogram: fix plot_log_percentile_histogram failing on config=`None`
Which is the default parameter, this could have been caught by `mypy` but `Optional[Dict]` is probably too much of a hassle to set up and make `mypy` happy.
Which is the default parameter, this could have been caught by `mypy` but `Optional[Dict]` is probably too much of a hassle to set up and make `mypy` happy.