Uninitialized value error returned by tails-restricted-network-detector

Originally created by @sajolida on #7689 (Redmine)

In WhisperBack report fe37e7be5155bf11291af6320571d173 someone experiencing Wi-Fi disconnections sent us log that included the following error:

Use of uninitialized value $state{“eth1”} in string eq at
/usr/local/sbin/tails-restricted-network-detector line 72.

Here eth1 is the Wi-Fi card.

Feature Branch: bugfix/7689-fix-tails-restricted-network-detector