Suggested values for reportprom are both ambiguous and false for deactivation

AFAIU, the reportprom setting should be set to nothing, in the provided default backupninja.conf, given the checks performed in the code.

However, currebtly, the default settings set (or suggested by the comments) in the .conf file will activate it: anything like "no" or "false" is not empty string, and thus leads to activating prometheus reporting.

Which leads to an error whenever prometheus isn't activated, on most boxes I guess: "Error: /var/lib/prometheus/node-exporter does not exist!"

See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1035758 for my initial report downstream.