lib: Don't render empty filters to SQL

Else we are presented with syntax errors.

fixes #9211

library/Icinga/Data/Db/DbQuery.php

@@ -166,12 +166,14 @@ class DbQuery extends SimpleQuery
                         $parts[] = $filterPart;
                     }
                 }
+                if (! empty($parts)) {
                     if ($level > 0) {
                         $str .= ' (' . implode($op, $parts) . ') ';
                     } else {
                         $str .= implode($op, $parts);
                     }
                 }
+            }
         } else {
             $str .= $this->whereToSql($filter->getColumn(), $filter->getSign(), $filter->getExpression());
         }