This can be useful for building grafana dashboards with dropdowns for multiple hosts
Or for managed instances of Prometheus where the user is not able to add labels via config
If an WMI query were to return an empty result set, trying to access the
first element in the result set would result in a `panic: runtime error:
index out of range` error.
add logic to explicitly check if the result set size is 0 and return an
error rather.
Fixes https://github.com/martinlindhe/wmi_exporter/issues/240
