Crash on updating moderators when the list has no owneers sympa-commu…

…nity#456

src/lib/Sympa/List/Users.pm

@@ -72,7 +72,7 @@ use constant _global_validations => {
         return if ($owner_domain =~ /^\s*$/);
 
         # calculate updated owner list, including deletions
-        my @owner = map { $_->{'email'} } @{$self->get('owner')};
+        my @owner = map { $_->{'email'} } @{$self->get('owner') || []};
         my $changes = $self->get_change('owner');
 
         #use Data::Dumper;