While the ConnectionsEstablished property in the
Win32_PerfRawData_Tcpip_TCP class is listed as a counter, real-world
metric values have been shown to increase *and* decrease.
Documentation for the property states "Number of TCP connections for
which the *current* state is either ESTABLISHED or CLOSE-WAIT" which
would imply the metric is a gauge.
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
