Fix disco_lvm_sudo when there's no VG at all

It was returning an empty string, which is not expected by Zabbix Server, so the item was marked as unsupported. It's now returning an empty JSON list

zabbix_scripts/disco_lvm_sudo

@@ -10,8 +10,11 @@ open STDERR, '>/dev/null';
 my $json;
 @{$json->{data}} = ();
 
+my @vg = eval { get_volume_group_list() };
+
+unless ($@){
   if ($what eq "volumes"){
-    foreach my $group (get_volume_group_list()){
+    foreach my $group (@vg){
       my %lvs = get_logical_volume_information($group);
       foreach my $lv (keys %lvs){
         $lv = ($lv =~ m!^/dev/$group!) ? $lv : "/dev/$group/$lv";
@@ -20,7 +23,7 @@ if ($what eq "volumes"){
     }
   }
   elsif ($what eq "snapshots"){
-    foreach my $group (get_volume_group_list()){
+    foreach my $group (@vg){
       my %lvs = get_logical_volume_information($group);
       foreach my $lv (keys %lvs){
         if (defined $lvs{$lv}->{allocated_to_snapshot}){
@@ -31,7 +34,7 @@ elsif ($what eq "snapshots"){
     }
   }
   elsif ($what eq "thin_pools"){
-    foreach my $group (get_volume_group_list()){
+    foreach my $group (@vg){
       my %lvs = get_logical_volume_information($group);
       foreach my $lv (keys %lvs){
         if (defined $lvs{$lv}->{allocated_pool_data}){
@@ -42,17 +45,17 @@ elsif ($what eq "thin_pools"){
     }
   }
   elsif ($what eq "groups"){
-    foreach my $group (get_volume_group_list()){
-        push @{$json->{data}}, { "{#LVMGRP}" => $group };
-    }
+    foreach my $group (@vg){
+      push @{$json->{data}}, { "{#LVMGRP}" => $group };                                                                                                                                                                }
   }
   else{
     print <<"EOF";
 
-Usage: $0 [volumes|snapshots|groups]
+Usage: $0 [volumes|snapshots|thin_pools|groups]
 
 EOF
+    exit 1;
+  }
 }
-
 print to_json($json);
 exit(0);