Don't include empty roles, slug & capabilities in UserListParams query_pairs

[oguzkocer]

// before
https://foo.com/wp-json/wp/v2/users?context=edit&slug=&roles=&capabilities=
// after
https://foo.com/wp-json/wp/v2/users?context=edit`

[jkmassel]

Seems good, but this seems like a good candidate for a unit test to me.

WDYT?

[oguzkocer]

@jkmassel I'll work on the unit tests for the users module in its own PR.