Per XEP-0030 section 3.1: 'if an entity has no associated items,
it MUST return an empty <query/> element.'
The client should display 'No service discovery items for X' when
receiving an empty result, not silently ignore it.
Previous version introduced in commit f28655c5c (2016-05-08) which added an early
return when child == NULL, preventing the message from being shown.