Report and exit if dig supportedOS returns nothing

Signed-off-by: Christian König <ckoenig@posteo.de>

automated install/basic-install.sh

@@ -190,6 +190,10 @@ os_check() {
 
         IFS=" " read -r -a supportedOS < <(dig +short -t txt ${remote_os_domain} @ns1.pi-hole.net | tr -d '"')
 
+        if [ -z "$supportedOS" ]; then
+            printf "  %b %bRetrieval of supported OS failed. Please contact support. %b\\n" "${CROSS}" "${COL_LIGHT_RED}" "${COL_NC}"
+            exit 1
+        else
           for i in "${supportedOS[@]}"
           do
               os_part=$(echo "$i" | cut -d '=' -f1)
@@ -208,6 +212,7 @@ os_check() {
                 break
               fi
             done
+          fi
 
         if [ "$valid_os" = true ] && [ "$valid_version" = true ]; then
             display_warning=false