Do not return empty relationships

If no relationships of a particular type are defined for a resource, the
key for that relationship should not be defined.

include/maxscale/monitor.h

@@ -322,7 +322,7 @@ json_t* monitor_list_to_json(const char* host);
  * @param server Server to inspect
  * @param host   Hostname of this server
  *
- * @return Array of monitor links
+ * @return Array of monitor links or NULL if no relations exist
  */
 json_t* monitor_relations_to_server(const SERVER* server, const char* host);