bp_profile_get_field_groups() does not set `fields` property for xprofile groups without fields

[DJPaul]

When using bp_profile_get_field_groups, you get an array of objects for each profile field group. Within each object, there's a property fields which is an array of objects for the fields in that group.

However, if a field group has no fields, this fields property is not set. It really should be set as an empty array for consistency in its return types.

[djpaul]

In 10935:

xprofile: always initialise fields property when fetching field groups.

When using bp_profile_get_field_groups, xprofile field groups without any fields did not have a fields property set in the return value. This changes initialises the property to an empty array, to increase the data consistency of the results.

Fixes #7154