Filter the network data before the query takes place.
Return a non-null value to bypass WordPress's default network queries.
The expected return type from this filter depends on the value passed in the request query_vars.
$this->query_vars['count'] is set, the filter should return the network count as an int.
'ids' === $this->query_vars['fields'], the filter should return an array of network ids.
Otherwise the filter should return an array of WP_Network objects.