Handle an empty local.list

Handle the case of an empty local.list file which would otherwise prevent the system from starting

Change-type: patch
Signed-off-by: Chris Crocker-White <chriscw@balena.io>

gravity.sh

@@ -184,8 +184,8 @@ migrate_to_database() {
 gravity_CheckDNSResolutionAvailable() {
   local lookupDomain="pi.hole"
 
-  # Determine if $localList does not exist
-  if [[ ! -e "${localList}" ]]; then
+  # Determine if $localList does not exist, and ensure it is not empty
+  if [[ ! -e "${localList}" ]] || [[ -s "${localList}" ]]; then
     lookupDomain="raw.githubusercontent.com"
   fi