Filters the network data before the query takes place.

Return a non-null value to bypass WordPress’ default network queries.

The expected return type from this filter depends on the value passed
in the request query vars:

• When $this->query_vars['count'] is set, the filter should return
  the network count as an integer.
• When '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.

Note that if the filter returns an array of network data, it will be assigned
to the networks property of the current WP_Network_Query instance.

Filtering functions that require pagination information are encouraged to set
the found_networks and max_num_pages properties of the WP_Network_Query object,
passed to the filter by reference. If WP_Network_Query does not perform a database
query, it will not have enough information to generate these values itself.

$network_data = apply_filters_ref_array( 'networks_pre_query', array( $network_data, &$this ) );