Do not set portal background in CSS if no value is configured (#1643)

lemonldap-ng-portal/lib/Lemonldap/NG/Portal/Main/Run.pm

@@ -793,13 +793,16 @@ sub sendHtml {
 
 sub sendCss {
     my ( $self, $req ) = @_;
-    my $s =
+    my $s = '/* LL::NG Portal CSS */';
+    if ( $self->conf->{portalSkinBackground} ) {
+        $s .=
             'html,body{background:url("'
           . $self->staticPrefix
           . '/common/backgrounds/'
           . $self->conf->{portalSkinBackground}
           . '") no-repeat center fixed;'
           . 'background-size:cover;}';
+    }
     return [
         200,
         [